Hallo,
OSX's priorities are rather a wish than a strong rule (somehow typical for all microkernel-derived architectures), so we can't guarantee the final resource consumption level. But still, have you tried to put the scanning daemon a bit deeper to background (= clicking background scan button)? Such custom-lowered scan priority can be later fixed in preferences to be used on each subsequent avast's launch.
It's quite questionable whether the scan should be most aggressive (many users want it this way to have their on-demand scan finished ASAP), or not so aggressive (other users like it silently running in the background, not disturbing their normal work or performance). that was also why the button "Background Scan" was added...
regards,
pc