I’ve been fighting with the problem of Lightroom Classic becoming unresponsive after returning to LrC from Nik (Silver Efex specifically). After trying to get support to help me (no success yet) I decided to do a lot of manual testing. At this point I’m convinced that the “updated” Nik apps that have the non-destructive edit feature are the problem. Specifically, they are outputting TIFF files that confuse LrC. If you let LrC wait long enough it will eventually sort things out. This takes roughly 30 seconds to 2 minutes for me depending on the megapixel size of the file and if non-destructive edits are turned on/off. I have a brand new M1 MacBook Pro 14" with 64GB so I imagine this problem is worse on slower hardware.
There are multiple threads on the forum that are really this exact same problem:
Basically when returning to LrC from the “updated” Nik apps, LrC gets stuck in some kind of loop trying to process the TIFF. Eventually it figures it out, but in many cases it’s simply faster to exit LrC and start it back up again.
I’m still hoping I can get support to send this to an engineer. So far no luck. The problem is 100% reproducible and I’ve sent them a video demonstrating it. Until then the best solution is to simply quit LrC after you exit a Nik app and restart LrC. The TIFF then works fine in develop mode with zero wait. Restarting LrC for me is actually faster than waiting up to 2 minutes for LrC to figure things out.