Catalina 10.15.5 Slow File Dialog
Solution 1:
I spent about a year working with Apple technical support on something that sounds just the same. The good news is that, assuming your problem is the same as mine, there is an easy fix. The challenge in diagnosis is that some interaction between iCloud and the Finder appears to be involved. iCloud's involvement is why problem persists across reinstalling the system, etc.
They fix was to delete either the dyld shared cache or the XPC cache. Unfortunately, I can't remember which it was and can't find my notes from then. But, since they are both just caches, there is no harm in deleting either or both.
The best way I found to do it is to download the Onyx app (free from https://www.titanium-software.fr/en/index.html). Under the "Maintenance" tab withing Onyx, you'll see the option to delete either or both of those caches. Do so, reboot and see if your problem is fixed. Once I did this, it fixed the problem entirely and it hasn't reappeared in over 2 years. There is probably a way to do this from the Terminal (indeed, Onyx is mostly just a wrapper over command line functionality), but Onyx worked great for me. I hope this helps you.
Solution 2:
You have ascertained that the problem does not occur in a new user account. That means something in your original user account Library is causing the problem.
It could be some software running in the background, like a Login Item, or LaunchAgent; or some preferences that are corrupt, or initiating something undesirable.
- In System Preferences > Users & Groups > Login Items, deselect everything, and test each item in turn.
- Check the contents of
<user>/Library/LaunchAgents. Remove anything there and restart.
If those don't improve matters, then it's possible some corrupt preferences may be causing the issue. You'll have to trawl through <user>/Library/Preferences, and test the problem after removing some of the com.apple.xxxx.plist files. I suggest removing them all; test the problem. If things work, then replace half of them; test the problem. If things don't work, remove half of the ones you put back. And so on.
As the problem relates to com.apple.appkit.xpc, you could try removing the com.apple.xpc.activity.plist files, and similar, first of all.
This may be tedious, and it's possible that the problem lies in some other part of the User Library folder, but each step narrows it down further and further. You could try deleting all user Caches, too.
The alternative is migrating to a new user account ("'slash and burn"). That's fine, but if you do that every time you get a user account problem, you'll end up with a string of user accounts, and files looking for different paths, and permissions issues, etc.