I'm having a similar or the same problem.
For days Avast has constantly used 30-40% CPU.
Avast Premium, current build, Win 10 64
After deleting and reinstalling as advised Avast now uses 55% CPU.
P.s. Disabling Cleanup and rebooting brought it back down to 30%.
P.p.s. And it's up at 54% again.
Programs running are Steam, Battle.net and Firefox, no downloads.
P.p.p.s. I have now put Avast into passive mode (which finally reduces CPU load back to near zero) because the fan was blowing harder than during peak summer but that is no long term solution obviously.