Use the tray icon of avast to pause the shields for 10 minutes. Open some programs (not on the Internet, so the chances you get to be infected are less) and at least "feel" the difference with the usual times 9you may want to do some action, like writing something in your word processor or whatever else you used to "feel" as slow. Now close the programs. When 10 minutes are gone, avast shields should start again.
When the shields are back ON again, repeat the same action (opening the same programs). Is there any important difference?
Usually, "some" difference is going to be, because of many factors, including avast scanning, Windows cache and others. But it should be not *that* much.
Free and used space (in percentage too) may be also a factor, and defragging may help (but only in *some* cases).
Another factor is the amount of processes running. For example, do you have many startup items? Can you delay or avoid some of those? Like, say, quicktime as an example, or adobe as another nonessential item that can be started manually when needed instead of autostartup.
As you see, there are many factors that could make your system "slow". Since you have not actually measured anything yet, we have no way to tell if avast is the source of the issue, or if avast is only *showing* you that you have some other problem (which is not the same as being the source of it), or even if avast is related in any way at all.