Usually, couple of things are going on:

  1. the MS Jet drivers (that avast uses to access its storage (mdb file)) tempt to periodically access the mdb file

  2. avast sometimes checks its ini file for changes

  3. obvious things like VRDB generator, scheduled scan etc.

For 1. and 2., I don’t really see how (even on a low end machine) this could considerably influence overall system performance.

Thanks
Vlk