There’s at least two issues. One is that it’s taking a long time to explode the group. I set my iMac and MacBoo Pro working on it a while ago, and neither have finished.
The other issue is that on the iMac the rainbow cursor stops spinning. I don’t know if it’s still working, and only the cursor is not being updated. Moving the cursor slightly gets it spinning again.
The cursor freeze, and some other freezes, seems to be a known issue with the M1 machines. Here’s an Apple and a Reddit topic about the problem:
https://discussions.apple.com/thread/252345825
I am fairly sure it will be a software issue, some of the people seeing the problem had their machine replaced, and still they see the problem.
I’m going to take a break for an hour, and will check both machines to see if they finished exploding that group.