Hello every one,
I can prove the issue is on Aspose side with a web test process we wrote, with this you can reproduce the issue when generating PDF, i.e. initial attempt is lengthy.
The way to apply this web process: deploy it in IIS, click Export PDF on WebForm1.aspx, it may take about 1 minute in first try, subsequent exporting would be faster. If you don't use IsHtmlTagSupported property,there is no this problem.
Problem is each time after IIS restarted or reached Idle Time-out(minutes) in IIS, the issue reoccurs! Deploying this web process in IIS can help reproduce the issue, only debugging it won't help.So I think this problem and used in the web project has a lot to do.