there are something you can do to reduce HDD activity from avast:
1/ disable transient caching (settings->file shield->advanced)
2/ add some following rules in file shield exclusion
https://imgur.com/mFAxKSe
Disabling transient caching won't reduce scanning in fact it is more likely to increase scanning.
The exclusions you give is more likely to put your system at risk and not advised.
1/ I'm talking the fact that after disabling transient caching, my disk activity dropped by a significant amount. Observed on many PCs with low specs. With transient caching, people complaint about avast slowing down their machines and after disabling it, their machines were much more responsive. Do you remember the incident of lscache.dat? I don't trust descriptions from avast because there are mistakes, I trust my own experiments
2/ those are selective exclusion, not full exclusion so it will significantly reduce avast's scanning but still preserve 95% of efficacy. I know what I'm doing and what I'm tweaking. In fact, behavior shield is not affected by this exclusion, just file shield so when something passes through all the shield, it will be blocked by behavior shield. I have been testing avast with zero-day malwares on VMs, I can say that these exclusion almost has zero effect on avast's protection capability, tested with and without exclusion. I know avast's weaknesses and strengths
