Thanks @tralph3,
It appears to be driven by some black magic.
Normally after installing Manjaro, I do this tweak that removes most of the tearing/lag on the desktop (not all): disable most of Compositor options and lean on Nvidia to handle composition. Perhaps this is part of the problem.
That said, on a fresh Manjaro install, with above tweak, I experience no issues in the game. That is (as per my testing yesterday) until I load the Global theme. I disable the Global Theme and it is back to normal.
I tried this approach on my current Manjaro and that does not appear to make a difference. Seems like more work to be done.
Looking for keyboard shortcuts for Compositor, i stumble upon “Kwin”? Is that it?