glCopyTexSubImage2D is very slow on Intel
For some context, this is about https://invent.kde.org/plasma/kwin/-/merge_requests/4211, so this is inside the compositor. On Wayland, using glCopyTexSubImage2D
causes considerable performance issues, making the compositor miss the pageflip deadline despite starting compositing ~16ms before vblank - this is very visible when moving the cursor around. Wrapping the same texture into an fbo and using glBlitFramebuffer
instead is very fast though.
It's possible that this also happens on GLX, but as the cursor is being updated asynchronously it's harder to tell whether or not compositor performance is bad.