Using RetroArch in Linux KVM mode and a CRT, I was able to get sub-16ms lag, and prove next-frame response time!
My specific test uses Pitfall! for the Atari 2600 on the Stella core.
This video was taken at 240fps on an iPhone, and slowed down further for demonstration purposes.
Easy for anyone to do, as long as you have a smartphone and a computer! No fancy wiring or lights required to test your own setup.
Just put the controller in front of your phone while taking a slow-mo video, and hit a button! Count slow-mo frames (use math to compare to 60fps at realtime) on a computer.
Graphics: Intel Iris 6200
OS: Ubuntu 17.04
Latest RetroArch Nightly
Random DisplayPort to VGA adapter
Extron VSC 500 (not required, but makes my VGA 240p signal look nicer)
VGA to circuit feeding CRT jungle chip
Sony Trinitron TV
Custom 1920x240p EDID via DisplayPort
RetroArch launched in KVM mode through basic command-line (not GUI)
Linux RetroArch settings:
Maximum Swapchain Images: 2
Frame Delay: as high as you can go without stuttering. 15 works for me for 8-bit systems
Super Famicom controller
Dual SNES controller to USB adapter – V2, configured to 1ms of lag. Website specs say 2ms minimum, but hey, 1ms was an option in the controls, and Linux reports 1ms poll time…