Probably other users experienced the same issue. Here is how I solved this.
I m using a MariaDB instance to store my video database. Over the past years I never had an issue with this when updating Kodi – except with the recent version 21. The installation went fine, a new database was created (MyVideos131) but then it took up to 5 minutes until Kodi was usable (responsive). In the debug logs this could be seen just as a time gap between two loglines – so no log output during that time.
As I am using 4 different KODI clients (WIN10, Librelec, Firestick and an Andriod TV) and all had the same issue after the update to version 21, I focussed on the common components.
Those are a TvHeadend server and the common database (only the Windows 10 client has write access here)
I turned out that the issue was related to the database. As there were some older MyVideosnn databases I deleted the old databases except the 2 latest but the result was even worse. At least I now knew that the database caused the problem.
Solution: delete all MyVideonn databases and let Kodi recreate one from scratch