Unless you change the hardware, a database server will always run at the same speed. However, individual process typically sprint, stop and wait, sprint, stop and wait, etc. It stops to wait for disk read or write operations to complete, it waits for remote procedure calls, it waits for locks to be released, etc. So, you need to analyze wait states during the period of time that the server appears not be getting as much work done. Determine what is waiting and why.
https://support.microsoft.com/en-us/kb/271509
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho