FPSMeter is an Ultrakill mod that adds a FPS counter. When installed, just press F3 anywhere in the game.
Put the downloaded .dll file into you BepInEx plugins folder or "UKMM Mods" if you use UKMM. Requires UKUIHelper to work. As with UKUIHelper, this has a high chance to work on other Unity games but i didn't test it.