IronXL Data-Loading Method Performance

Updated:June 28, 2026

When you populate a large worksheet in IronXL, the loading method you pick has a big effect on how long the file takes to generate. The benchmark below compares three approaches against the same dataset: 20,000 rows by 55 columns, each written to disk as an .xlsx file.

Test Overview

Every test populates the same data and writes the result to disk, recording both the duration in seconds and the resulting file size.

Adding data cell by cell: ~24 sec, 3094 KB
Loading from a DataTable: ~13 sec, 3094 KB
Loading from CSV: ~9 sec, 3094 KB

The file size is identical across all three. Only the time to build the file changes.

The Methods

Option 1: Adding Data Cell by Cell

This is the slowest of the three. Each cell is assigned individually through nested loops.

string[] fruits = { "Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                        "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                        "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                        "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                        "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                        "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                        "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                        "cherry", "nectarine", "Butternut squash"};
var workbook = WorkBook.Create(ExcelFileFormat.XLSX);
var worksheet = workbook.CreateWorkSheet("Fruits");
var columnsTitles = GetExcelCellLetters(fruits.Length);
for (int col = 0; col < columnsTitles.Count; col++)
{
    string columnLetter = columnsTitles[col]; // To avoid interpolation that can cause performance issues in large loops because it build string objects in every loop
    for (int row = 1; row <= 20000; row++)
    {
        worksheet[columnLetter + row].Value = fruits[col];
        //worksheet.Rows[row].Columns[columnLetter].Value.ToString();
    }
}
workbook.SaveAs("C:\\Temp\\ManualCell-by-CellAssignment.xlsx");
private static List<string> GetExcelCellLetters(int iMaxNum)
{
    string[] alphabet = { string.Empty, "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };
    IEnumerable<string> lst = (from c1 in alphabet
                               from c2 in alphabet
                               from c3 in alphabet.Skip(1)
                               where c1 == string.Empty || c2 != string.Empty
                               select c1 + c2 + c3).Take(iMaxNum);
    return lst.ToList();
}

string[] fruits = { "Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                        "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                        "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                        "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                        "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                        "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                        "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                        "cherry", "nectarine", "Butternut squash"};
var workbook = WorkBook.Create(ExcelFileFormat.XLSX);
var worksheet = workbook.CreateWorkSheet("Fruits");
var columnsTitles = GetExcelCellLetters(fruits.Length);
for (int col = 0; col < columnsTitles.Count; col++)
{
    string columnLetter = columnsTitles[col]; // To avoid interpolation that can cause performance issues in large loops because it build string objects in every loop
    for (int row = 1; row <= 20000; row++)
    {
        worksheet[columnLetter + row].Value = fruits[col];
        //worksheet.Rows[row].Columns[columnLetter].Value.ToString();
    }
}
workbook.SaveAs("C:\\Temp\\ManualCell-by-CellAssignment.xlsx");
private static List<string> GetExcelCellLetters(int iMaxNum)
{
    string[] alphabet = { string.Empty, "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };
    IEnumerable<string> lst = (from c1 in alphabet
                               from c2 in alphabet
                               from c3 in alphabet.Skip(1)
                               where c1 == string.Empty || c2 != string.Empty
                               select c1 + c2 + c3).Take(iMaxNum);
    return lst.ToList();
}

Imports System
Imports System.Collections.Generic
Imports System.Linq

Module Module1
    Sub Main()
        Dim fruits As String() = {"Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                                  "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                                  "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                                  "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                                  "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                                  "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                                  "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                                  "cherry", "nectarine", "Butternut squash"}
        Dim workbook = WorkBook.Create(ExcelFileFormat.XLSX)
        Dim worksheet = workbook.CreateWorkSheet("Fruits")
        Dim columnsTitles = GetExcelCellLetters(fruits.Length)
        For col As Integer = 0 To columnsTitles.Count - 1
            Dim columnLetter As String = columnsTitles(col)
            For row As Integer = 1 To 20000
                worksheet(columnLetter & row).Value = fruits(col)
            Next
        Next
        workbook.SaveAs("C:\Temp\ManualCell-by-CellAssignment.xlsx")
    End Sub

    Private Function GetExcelCellLetters(iMaxNum As Integer) As List(Of String)
        Dim alphabet As String() = {String.Empty, "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"}
        Dim lst = (From c1 In alphabet
                   From c2 In alphabet
                   From c3 In alphabet.Skip(1)
                   Where c1 = String.Empty OrElse c2 <> String.Empty
                   Select c1 & c2 & c3).Take(iMaxNum)
        Return lst.ToList()
    End Function
End Module

$vbLabelText $csharpLabel

Note the cached columnLetter: building the column reference once per column instead of interpolating it inside the inner loop avoids allocating a new string on every iteration. Reserve this method for cases that need genuine per-cell logic or styling.

Option 2: Loading from a DataTable

A solid middle ground, and significantly faster than manual cell insertion. Build a DataTable in memory, then hand the whole thing to LoadWorkSheet.

string[] fruits = { "Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                        "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                        "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                        "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                        "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                        "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                        "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                        "cherry", "nectarine", "Butternut squash"};
var workbook = WorkBook.Create(ExcelFileFormat.XLSX);
DataTable table = new DataTable("Fruits");
foreach (string fruitName in fruits)
{
    table.Columns.Add(fruitName);
}
int rowCount = 20000;
for (int i = 0; i < rowCount; i++)
{
    table.Rows.Add(fruits);
}
workbook.LoadWorkSheet(table);
workbook.SaveAs("C:\\Temp\\LoadDataTableDirectly.xlsx");

string[] fruits = { "Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                        "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                        "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                        "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                        "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                        "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                        "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                        "cherry", "nectarine", "Butternut squash"};
var workbook = WorkBook.Create(ExcelFileFormat.XLSX);
DataTable table = new DataTable("Fruits");
foreach (string fruitName in fruits)
{
    table.Columns.Add(fruitName);
}
int rowCount = 20000;
for (int i = 0; i < rowCount; i++)
{
    table.Rows.Add(fruits);
}
workbook.LoadWorkSheet(table);
workbook.SaveAs("C:\\Temp\\LoadDataTableDirectly.xlsx");

Imports System.Data

Dim fruits As String() = {"Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                          "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                          "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                          "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                          "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                          "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                          "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                          "cherry", "nectarine", "Butternut squash"}

Dim workbook = WorkBook.Create(ExcelFileFormat.XLSX)
Dim table As New DataTable("Fruits")

For Each fruitName As String In fruits
    table.Columns.Add(fruitName)
Next

Dim rowCount As Integer = 20000
For i As Integer = 0 To rowCount - 1
    table.Rows.Add(fruits)
Next

workbook.LoadWorkSheet(table)
workbook.SaveAs("C:\Temp\LoadDataTableDirectly.xlsx")

$vbLabelText $csharpLabel

LoadWorkSheet ingests the entire table in one call, which is why it beats writing each cell by hand. Reach for this when your data already lives in structured memory.

Option 3: Loading from CSV

Fastest overall, and the natural choice for flat, delimited data. Write the rows to a CSV file, then read it back with LoadCSV.

string[] fruits = { "Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                        "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                        "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                        "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                        "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                        "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                        "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                        "cherry", "nectarine", "Butternut squash"};

            var sb = new StringBuilder();
            // Step 1: Add header row
            sb.AppendLine(string.Join(",", fruits));
            // Step 2: Add 20,000 identical rows
            string rowData = string.Join(",", fruits); // same as headers
            for (int i = 1; i < 20000; i++)
            {
                sb.AppendLine(rowData);
            }
            File.WriteAllText("C:\\Temp\\csvfile.csv", sb.ToString(), Encoding.UTF8);
            var workbook = WorkBook.LoadCSV("C:\\Temp\\csvfile.csv");
            workbook.SaveAs("C:\\Temp\\LoadFromCSV.xlsx");

string[] fruits = { "Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                        "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                        "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                        "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                        "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                        "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                        "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                        "cherry", "nectarine", "Butternut squash"};

            var sb = new StringBuilder();
            // Step 1: Add header row
            sb.AppendLine(string.Join(",", fruits));
            // Step 2: Add 20,000 identical rows
            string rowData = string.Join(",", fruits); // same as headers
            for (int i = 1; i < 20000; i++)
            {
                sb.AppendLine(rowData);
            }
            File.WriteAllText("C:\\Temp\\csvfile.csv", sb.ToString(), Encoding.UTF8);
            var workbook = WorkBook.LoadCSV("C:\\Temp\\csvfile.csv");
            workbook.SaveAs("C:\\Temp\\LoadFromCSV.xlsx");

Imports System.IO
Imports System.Text

Dim fruits As String() = {"Apples", "Apricot", "Banana", "Blackberry", "Blueberry", "Boysenberry", "Canary Melon", "Cantaloupe",
                          "Currants", "Dates (tree dried only) Dragon Fruit", "Durian (purchase cut) Figs", "Gooseberry", "Grapes", "Grapefruit",
                          "Lemon", "Lime", "Loganberry", "Longan", "Loquat", "Lychee", "Mandarin", "Mango",
                          "Blood Orange", "Papaya", "Passion Fruit", "Peach", "Pear", "Persimmon", "Pineapple", "Plum", "Honeydew",
                          "Rambutan", "Starfruit", "Tamarind", "Yuzu", "Açaí", "Abiu", "Ackee", "Breadfruit", "Cempedak",
                          "Cherimoya", "Buddha's Hand", "Citron", "Finger Lime", "Kumquat", "Pomelo", "Tangelo", "Ugli Fruit",
                          "Galia Melon", "Kiwano (Horned Melon)", "Mouse Melon", "Muskmelon",
                          "cherry", "nectarine", "Butternut squash"}

Dim sb As New StringBuilder()
' Step 1: Add header row
sb.AppendLine(String.Join(",", fruits))
' Step 2: Add 20,000 identical rows
Dim rowData As String = String.Join(",", fruits) ' same as headers
For i As Integer = 1 To 19999
    sb.AppendLine(rowData)
Next
File.WriteAllText("C:\Temp\csvfile.csv", sb.ToString(), Encoding.UTF8)
Dim workbook = WorkBook.LoadCSV("C:\Temp\csvfile.csv")
workbook.SaveAs("C:\Temp\LoadFromCSV.xlsx")

$vbLabelText $csharpLabel

LoadCSV parses the delimited text in bulk, which keeps it ahead of the other two approaches.

Conclusion

To get the best performance from IronXL on large datasets:

Use CSV: the fastest path for raw data imports when the source is already flat.
Use a DataTable: ideal when the data already exists in structured memory.
Avoid cell-by-cell writing: only worth it when you need per-cell logic or styling.

All three produce the same file size. The loading method is what dramatically changes how long the file takes to generate.

Curtis Chau

Chat with engineering team now

Technical Writer

Curtis Chau holds a Bachelor’s degree in Computer Science (Carleton University) and specializes in front-end development with expertise in Node.js, TypeScript, JavaScript, and React. Passionate about crafting intuitive and aesthetically pleasing user interfaces, Curtis enjoys working with modern frameworks and creating well-structured, visually appealing manuals.

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