Hi yihengsun,
Thank you for your interest in Syncfusion Products
Essential DocIO serializes a large amount of data to create this document, so that it takes few more time to generate the document. It is very difficult to bring the time taken under 3 seconds. We are sorry for the possible inconvenience caused.Here is the sample for your reference.
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=F91910-1793480140.zipPlease let us know if you have any questions.
Regards,
Poornima