Just thought I'd mention this thread, were I've made a lot of input lag tests on RetroArch (and even a real SNES on a CRT) using controllers rigged with LEDs and a high speed camera. The first post contains a lot of measurements, but also links to my other posts later in the thread which contain interesting info. Hopefully this gives you some ideas.
https://forums.libretro.com/t/an-input-lag-investigation/4407
https://forums.libretro.com/t/an-input-lag-investigation/4407