"I have switched from LRC 10.2 (Intel) to LRC 10.3 (native, Apple silicon) but since then everything is incredible slow."
LR may be rebuilding Library previews, which can happen when you move to a new machine, depending on how you did it. To check, run Activity Monitor, click the CPU tab, click the %CPU column to sort by %CPU in descending order. Is LR using more than 50% CPU constantly?
Also, verify that LR isn't indexing faces in background. Click the Identity Plate in the upper-left corner to see its poorly hidden menu:
If you've verified that LR isn't building previews and indexing faces, then post a problem report in the official Adobe feedback forum, where Adobe wants all product bugs and feedback:
Be sure to include the first ten lines of the menu command Help > System Info. Provide all the details, rather than linking back to this thread -- experience shows that Adobe developers tend not to follow the links. Product developers read everything posted there, but rarely participate here, which is primarily a user-to-user forum.