Any suggestion? We're forced to disable shields occasionally (when the slowdown occurs) what makes our computers vulnerable...
Although I think only Avast developers would be able to answer (if any), you could try temporarily disabling only part of the shields to narrow down the problem, instead of disabling "all" shields at once.
For example, once the slowdowns start bothering, stop/pause the script shield only, and test if there is any improvement. Then enable the script shield again, and see if this triggers the slowdowns again.
Repeat the tests with each individual shield (although some are interconnected/dependend on each other). Once the specific problematic shield(s) is (are) identified, repeat the tests with the suspicious shield(s).
Once confirmed, report your test results here for the developers.