Disabling the FSS could put your system at risk and isn't advisable, you could just disable the persistent cache, though I think this is totally unrelated.
The 'Speed up scanning by using the persistent cache' should be on by default I believe. However, when first enabled the 'Store data about scanned files in the persistent cache' it would have an impact on the scan (as it says in the notes below the option), but that should be the sum of it. It shouldn't extend into the normal use of avast.
Your system spec is more than up to the task and really shouldn't see this kind of impact to the persistent cache, which as I said shouldn't extend into normal use.
How long have you had avast installed ?
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security software do you have installed (anti-spyware, firewall, etc.) ?