Premiere Pro 2024 is really struggling

I am trying to use Premiere Pro 2024 on my 2020 Mac Pro and facing big problems. Playback is jittery, the response time is awful. I hit space and have to wait about 8 seconds before anything plays. 

The same system works ok with Premiere Pro 2021. Any version later than this really struggles. 

I have 84GB of Ram dedicated to running Premiere Pro. In theory, my machine ought to be super powerful: 3.2GHz 16-Core Intel Xeon W processor,  AMD Radeon Pro Vega 11 32GB Graphics. All software up to date.

I shoot on the Canon C300 mark 3 and Canon C70 cameras in 4k XF-AVC Long gop. As I say, this set up has worked fine in Premiere Pro 2021 but later versions have all been awful. As a last resort, does anyone have expereince of using a 2020 Mac Pro with Premiere Pro or does anyone have any suggestions for how to fix this issue please?