Something has changed in 2020 that has killed playback speed

Dear Adobe,

I'm running Catalina 10.15.1 on my iMacPro with Vega 56 graphics card. 

I'm also running PP 2019 and PP 2020 on the same machine.

When I put a simple 4K H264 .MXF file created on a Sony FS5 camera on an HD timeline, both versions play the clip smoothly.

But when I speed the clip up 200%, it is silky smooth on PP2019 BUT on PP2020 it's almost unplayable and very jerky.

How can this be? I'm using Metal as recommeded on both installations of PP and I've tried using the OpenCL option too. No difference. The same behaviour.

How can you sell me software like this and expect it to work in a production environment with silly bugs in it like this???!!

Please respond...