@sam Also use https://developer.gnome.org/documentation/tools/inspector.html to check the global properties of the app's graphics stack, and see if you see any mention of "LLVMpipe" in there. If so, that would mean the app/GTK is somehow running in software rendering mode, without any GPU assistance, everything on the CPU, and that's terribly slow. Also watch out if it's running Vulkan (instead of GL) on a GPU that is not actually Vulkan…