I have this exact problem which also began recently.
There are two instances of runonce which are not in the registry, a 32-bit one and a normal 64-bit one.
The initial instance of the secure browser checker just hangs and causes a black screen for the longest time.
If I kill the runonce instances the boot then speeds up.
This is ridiculous, why can't the browser be updated as part of AVAST program update process?
Edit.
So I did find the runonce registry entry for this, it was under syswow64 node, basically do a search for "avast_browser_setup_checker.exe".
Removed that registry entry and my logins are once again fast, instead of waiting on one occasion over 7 minutes with a black screen whilst the browser setup checker was misbehaving.
This seriously needed lots of internal testing before it was pushed out, and again should be part of the well established and efficiently working Avast software and virus definitions update process. This solution is just bloatware.
Edit.
Spoke too soon, it's back again and once again a long login with a black screen, seriously this is just garbage.
Will be following fumma's tip and see if that solves this problem, Avast developers please note causing a black screen and hanging up the user login process should never be considered acceptable.