What version of QVS are you using? We have found customers that are running 11.2 SR2 and 11.2 SR3 experience similar issues. We have upgraded them to 11.2 SR4 and those performance issues have been resolved. Whenever you post Server specs, be sure to include the QVS version you are using. It is important.
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I got the similar symptoms for some customers, both in reloads an distributions. The task execution time became much higher during the week or during the time depending of the amount of tasks.
I resolve that by restarting distribution service every day in a batch job until i got a cal from QLiktech support explaining that is was related to a bug in Microsoft .net framwork. And the fix they recommended resolved the problem.
here you are the information: