Create a file with 1 000 000 rows and 10 columns and print out how long it takes.

Create-1_000_000.cs

var sw = Stopwatch.StartNew();
using var wb = new XLWorkbook();
var sheet = wb.AddWorksheet();

// Lazy created data
var data = Enumerable.Range(0, 1_000_000).Select(_ =>
{
    return Enumerable.Range(0, 10).Select(_ => "Hello world");
});

sheet.Cell("A1").InsertData(data);

sw.Stop();
Console.WriteLine($"Insert data: {sw.ElapsedMilliseconds}ms");
sw.Restart();
wb.SaveAs(@"1_000_000.xlsx");
sw.Stop();
Console.WriteLine($"Save data: {sw.ElapsedMilliseconds}ms");