[ul][li]An extreme amount of disk activity can sometimes cause Opus to get bogged down trying to process all the events (particularly if the folder tree is turned on, although that was made much better in 10.5.2.0).
The Changes to folders are not being detected guide has some debugging steps which are usually used to see if disk activity is being detected at all, but can also be used to see if there is an excessive amount of activity. (Excessive in this case would mean the list of events scrolling non-stop, faster than you can read them, for a few minutes.)
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[li]The problem may also be caused by a shell extension.
These two guides have suggestions on tracking down shell extension problems (the first one is probably your best bet from what you've described):
Crash, exit or high CPU usage when viewing certain directories
Crash, exit or high CPU usage when right-clicking certain files
There have been issues with some cloud storage shell extensions recently, so disabling those is a good first step which may save time. (We have a change coming which may resolve some of the cloud storage extension issues, too.)[/li][/ul]