Premiere 2020 - Preview in program monitor very very poor, slow, useless

Hi Premiere people

I had to upgare to 2020 version for compatibility issues with my co workers and the project got f**ed. The preview doesn't work at all. It was all ok on the 2019 version.

I've cleared cache, changed render to software only (this improved things a tiny bit but not much). Basically premiere struggles to play a 1024*576 proxy at 1/4 of the quality!

Any recommendation? I am on a mojave mac 2018 with 16GB of RAM, with Radeon Pro 560X 4 GB. Premiere 14.0.1

Hope to get this sorted otherwise it's a wasted day in the edit, and not great for a 30gbp/month subscription.

Thanks

Silvia