In version 15.2 in Full Screen I cannot get smooth playback using the renderer Mercury Playback Metal Engine, where frames are dropped. However, using Mercury Playback OpenCL no frames are dropped. I have a 2017 IMac Pro 3.2 GHz 8-Core Intel Xeon W, Radeon Pro Vega 56 8 GB, and 32 GB memory. I also have Blackmagic EGPU, and an 8TB Thunderbolt 3, two disc RAID.
Up unti today I've been reluctant to post this as OpenCL works just fine, however in testing the beta version Open CL is removed and Mercury Playback Engine does not result in smooth playback. I would hate to upgrade to a future version and be stuck with this issue.
It seems to be the Blackmagic EGPU. Disconnect it and playback is fine in Metal. Select OpenCL and the activity monitor, GPU history shows that it switches from the EGPU to internal GPU. Hmm. That's dissapointing.
Any particular reason you're using an eGPU on a system with Radeon Pro Vega 56 8 GB graphics? The eGPU is more for an Intel Mac mini that only has Intel UHD Graphics 630. Although, the integrated graphics of the Mac mini actually performs rather well with Metal enabled.