How do different data-loading methods in IronXL compare in terms of performance?

Summary of Each Method

1. Adding Data Cell by Cell

This approach manually assigns data to each individual cell using nested loops.

• Performance: Slowest of the three

Code:

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();
}

Result:

2. Loading from DataTable

 One of the two fastest options. From v2026.10.1 it's as fast as CSV or faster, and much faster than manual cell insertion. 

• Performance: Significantly faster than manual cell insertion

Code:

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");

Result:

3. Loading from CSV

 One of the two fastest options, and the natural choice when your data is already flat and delimited. 

• Performance: Significantly faster than manual cell insertion

Code:

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");

Result: