Yes, there has been a slowdown with reviews since the challenge, and this can be due to burnout and fatigue, but there is a more likely reason. One of the biggest reasons was due to a bug in the system causing reviewers’ ratings to decrease dramatically. While it had been happening since April, it wasn’t until the challenge that it was noticed by many reviewers, with many having their ratings go from Great to Poor after only a few reviews.
While this issue has been resolved, some reviewers haven’t come back, as they may not know the issue has been resolved.
It also could be where your review area is for your nominations, as some areas have more reviewers than others, so it’s not uncommon for it to take days, weeks, months, even longer for a resolution to be reached. There really isn’t any guaranteed timeframe for when enough votes will be received.
However, there is one thing to consider, and that would be upgrades. If you have any upgrades, you may want to consider using them to get to a resolution faster, as it opens your nominations up to a wider reviewer area.