What is your version, the very latest is 4.7.981, this version has lowered the priority inthe update process. However the standard shield (ashServ.exe) is the main scanning engine and it needs to be able to intercept files to be scanned before they are opened.
If the the priority were reduced it may allow malware to get established, the idea of the high priority is also designed to quickly scan and hand back to other applications of a lower priority.
Can you tell us about your system, CPU, RAM, etc. ?
What sensitivity have you got the Standard Shield set at (Normal is the default, high will scan more files) ?
What is it that you are doing that is being slowed ?
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have that might have an impact, e.g. anti-spyware, Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, Spyware Doctor, etc. ?
The reason I ask all these questions is, avast by comparison to other AVs is light on resources and I see no noticeable impact on what I do and my system isn't any speed machine.