@SRSR333
The compositing lag you observe when you only have a Reverse PRIME display is caused by a limitation in X Present, it can’t sync to PRIME sinks. It works with NVIDIA-based PRIME Sync (non-reverse) because the NVIDIA driver implements its own mechanism to allow vsync, but when the NVIDIA GPU is the sink, as in Reverse PRIME, we have to rely on what the server supports. There is some work being done upstream on this: Sync present to slave outputs (!460) · Merge requests · xorg / xserver · GitLab. Once that is implemented, some further work will be required on the NVIDIA side to support it. Unfortunately, the only current workaround is to have a non-Reverse PRIME display set as the primary in RandR, or to disable vsync.
@pobrn
Thanks for reporting that issue, I was able to reproduce it and a fix is in progress, tracked under internal bug number 3115486.