Wayland reported game FPS doesn't match perceived FPS

For about as long as I’ve been using Plasma, I’ve experienced a mismatch between what games (or overlays like Mangohud) report fullscreen game FPS as being versus what it actually appears to be if VSync is disabled (VRR does not appear to resolve the issue). I’ve seen this in multiple native and proton games, but probably the most prevalent would be War Thunder, where a reported ~350+ FPS can at times look as low as 20-30 FPS.

I’m not actually certain that this is a visual stutter as much as it might be out-of-order frames, since it visually appears to jump backwards if paying close enough attention. I also don’t really know what debugging information I can provide from Plasma itself, but the issue is one I’ve experienced with a few different configurations:

  • My old system, nvidia + Intel on Arch w/ Plasma 5.24 up until Plasma 5.27
  • My new system, AMD + AMD on Arch w/ Plasma 5.27 and Plasma 6.
  • The same new system, on Fedora 40 w/ Plasma 6… but not Fedora 39 with Plasma 5.27.

Additionally, the issue only occurs in Plasma Wayland sessions, and does not seem to occur in Plasma X11 or any other DE/WM running wayland (tested Hyprland and Gnome Wayland). The fact that Fedora 39 did not experience any issues while Arch, running the same Plasma 5.27, did, makes me think this might be something core to Wayland itself that was more up-to-date on Arch for a long time than it was on Fedora.

Enabling VSync does seem to fix the issue at the cost of quite noticeable input latency, but when it is enabled the system seems much more sensitive to actual FPS drops.

Really appreciate any input - or if I just need to file a bug, what information might be useful to include here.