Only a data point for reference.
I am on Avast8 for most machines and Avast9 on one other…all W7 64bit SP1 with 4/8GB RAM, all I5 cores.
I have MBAM Pro running with website blocking.
The MBAM service takes up 50MB but little CPU resource.
I have nothing in exclusion lists and have both Avast & MBAM Pro run nightly scans at different times.
I’ve had this setup for couple months and on the Avast8 systems there is no impact.
I’ve not spent much time on the Avast9 machine since it is a test mode one for me…I’ll check but seems on most apps to be “snappy”.