For ANY performance test, ANYTHING running can affect the results.
One thing is that a certain (benchmark) program might have some incompatibility with Avast (or with any other AV tool).
A very different thing is that you want to run that (benchmark) program without ANY interference or influence from Avast (or from ANY running program, for that matter). For this second situation (a "lab" test"), you would still have some influence coming from "anything" running (services, drivers, processes...).
So, if you don't want ANY influence (for the purpose of the benchmark), then I'm not sure you can use ANY kind of resident security tool, at all. You either uninstall "anything and everything", or you run your benchmark under Windows Safe Mode (which by itself gives you different results than when running the same benchmark in a "pure and clean" recently installed Windows Normal Mode.
If you could provide a more specific detail information about what is exactly not working or what is "not compatible", then maybe we can help you better.