For a start we (avast users) don’t know exactly what you mean by check/scan directly or for that matter what ESET does.

Scanning the https secure encrypted traffic directly (as I mentioned isn’t the issue) is unlikely to detect anything because its encrypted form differs completely from it unencrypted state.

If there is some other intervention to be able to scan the unencrypted form before it is displayed/run in the browser, then that may slow browsing depending on how that is done.