File a support case.
This is something that should be closely studied to figure out exact cause.
I've seen few customer cases with this description however there could be few underlying causes for this. Support should help.
hey,have you found any offical documents to discuss this issue? or any documents to support that restart can make the server faster. I have face the same issue..
I'm guessing some sort of scanning was running in the background and the restart stops it. This link may be helpful: http://kb.tableau.com/articles/howto/improving-performance-by-using-antivirus-exclusions
Improving Performance by Using Antivirus Exclusions | Tableau Software ; also, make sure Tableau is the only app running on the server, don't run others like SQL server etc. on the same machine.