I have started playing Witcher 3 in immersive mode with sharpening set to 2.0. When I start the game Vorpx warns me with a red message for too low FPS but when I am in the game and look at the FPS using FpsVR I see almost a constant 90 FPS.
Use the built in FPS counter (ALT+F). vorpX has two render-pipelines running in parallel: the game itself and a second one which renders the final image to the headset, both can have different frame rates. The built in FPS counter displays both.
In cinema/immersive mode you can get away with rather low game FPS. Even 25-30fps may work fine in that case if the game isn’t a super fast shooter.
Because in a handful of games drawing the vorpX menu has a severe impact on performance, so to avoid confusion the FPS counter is not shown in the menu.
There are already too many options in the menu, so to reduce menu clutter a few less important things are hotkey only. You can map ALT+F to your controller if you consider it important enough to be available anytime while you play standing.