How to find out the activity which increasing the response time of the Sql Server?
I highly recommend using Adam Machanic's excellent diagnostic stored procedure sp_WhoIsActive.  I've got an sp_WhoIsActive tutorial on my blog.  Run it during the slow time window as I describe in the video, and it'll show the active queries, what they're waiting on, and their execution plans.

Keep in mind, though, that this is more of a DBA tool than a "here's-your-problem" tool.  This is where a database administrator usually has to step in and do diagnostic work.  If you're confused by the output of sp_WhoIsActive, check out the book Troubleshooting SQL Server.