Created Unassigned: it makes me wonder why it's so solw when i export excel added cell style [12823]

ICellStyle cellstyle = workbook.CreateCellStyle();
//border
cellstyle.BorderBottom = BorderStyle.THIN;
cellstyle.BorderLeft = BorderStyle.THIN;
cellstyle.BorderRight = BorderStyle.THIN;
cellstyle.BorderTop = BorderStyle.THIN;
//font
IFont font = workbook.CreateFont();
font.FontName = "Aral";
font.Boldweight = (short)FontBoldWeight.BOLD;
cellstyle.SetFont(font);

I write above.When i export 7000 records to excel,It use 1h.But when i remove the style ,It only use 5s.
It's so unbelievable.

please help me