Playback lag on 16" M1 MacBook Pro (2021) with Slog 3 files

Just bought a open box M1. Edited a video shot in ProRes the other day with no problems. Today I tried to start working on a video shot on a Sony a7siii, in 4K 10-bit Slog 3 in H.264. After I synced the clips, I couldn't even play the video back at 1/4 quality. I tried separating the clips into smaller groups so it would be less clips to have to playback at once but it still was super laggy. I tried several things people suggested on here and on YouTube, but nothing is working. It's impossible to edit a music video like this.