TCPDF slow download in IIS

I hope you guys can help me here. I have a PDF 开发者_如何学编程report generated using TCPDF but the problem is when downloading it gets delayed in "Starting Download" or sometimes in 85% downloaded or sometimes in 100%. I tested it in Apeche and it works as it should but when uploaded in the live server (IIS), it has the problem.

BTW, its not the execution of the code.. the lag happens after code execution (downloading process)

Thanks a lot..