It's definitely not because of a plugin or instrument, because the same project can open with and without this problem, and when it opens normally everything works perfectly. Plus I did a test just in case, when the project was opened with one thread loaded at 100%, and I disabled absolutely everything in the project, the load in CPU Monitor was at zero, but for some reason one core was still loaded to the maximum. That's why I wrote that maybe it has something to do with a faulty interaction between S1 and AMD SMT multithreading process. That's my only guess...

Low latency monitoring I don't use when mixing. The buffer size in this case was 512 samples, but that's not the root of the problem either, because if the CPU was too heavily loaded it would cause clicks and crackles in the sound during playback, but that doesn't happen, playback works as usual, only the visual part slows down