Well that's weird, but then it also looks like a Xorg or Wayland issue - or a desktop environment issue. I believe we found out at some point that under Gnome Shell, vertical synchronization was basically forced by the desktop environment because it would take our raw frames and push them at vsync - regardless of whether we wanted that.
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Try adding this in ~/.config/Epic/HeliumRain/Saved/Config/LinuxNoEditor/Engine.ini.
This should effectively disable framerate smoothing, and ensure you can get higher framerate.
Well this is weird. We definitely build with SF_VULKAN_SM4 (and SF_VULKAN_SM5). In fact, we didn't change a thing !
Could you take a quick look in the game folder and confirm that /HeliumRain/Content/Paks exists, with HeliumRain-LinuxNoEditor.pak as a ~3.5GB file ?