Create a file with 250 000 rows and 15 columns and print out how long it takes.

ClosedXML-250_000.cs

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

// Lazy created data
var data = Enumerable.Range(0, 250_000).Select(_ =>
{
    // Always create new string, like database provider would
    var text = Enumerable.Range(0, 10).Select<int, object>(_ => new string("Hello world"));
    var numbers = Enumerable.Range(0, 5).Cast<object>();
    return text.Concat(numbers);
});
sheet.Cell("A1").InsertData(data);
sw.Stop();
Console.WriteLine($"Insert data: {sw.ElapsedMilliseconds}ms");

sw.Restart();
wb.SaveAs(@"large_250_000.xlsx");
sw.Stop();
Console.WriteLine($"Save data: {sw.ElapsedMilliseconds}ms");