I came across this while researching input latency for my gaming setup. I just bought a VIZIO p55-f1 for it's dedicated low latency HDMI5 port touting 10.5ms 1080p120Hz. While it's not as good as 240Hz computer monitors, it's the best you can get for a big screen at the moment. Price was good too. I'm still waiting on it to be delivered, so I'll have to test on my current TV for now.
Now, I wasn't expecting a miracle. I figured it might help but at the cost of something. I didn't believe it could compare to a CRT. No way. I just DL'ed Retroarch 1.7.5 and it's frack'n awesome. When I turn the Run-Ahead Feature on and follow the steps in the video above, it takes 2 frames to completly eliminate the lag, but otherwise I haven't noticed any performance hit in the few minutes I've tested it. It does cause audio glitches, but the second instance feature fixes that (this may be a problem for slower devices like the Pi or classic consoles). WOW...I'm blown away! It feels like I'm playing on my CRT. I Know because I have one right underneath my big screen comparing the same game side-by-side....
...Gasp!
Let me put this another way:
I have my Super Mario World on my OG SNES running on a CRT. I am using 8BitDo bluetooth controllers for both. I'm using the DIY for SNES controllers on my OG, but I'm using the SFC30PRO for the PC. Same lag for each. I notice very little lag for the OG SNES with the bluetooth, it's acceptably low IMO. I did break out the OG Super Famicom Controller just to be sure (My favorite). It does feel more responsive than the 8bitdo wireless controller. I mean it's not huge, but it's just to me (it could be the placebo effect, I'm not sure).
I just spent $700 on a TV prioritizing low input lag. With one update Libretro redifined the limits of what I thought was possable via emulation. I thought we were going to have to accept that emulation take time to process, framebuffer, and etc. Add that to modern TV's inherent Input lag and I didn't think CRT like performance was possible.
This is a GREAT feature!!! Easily one of the best features Retroarch has released yet. Now NES runs ok on retropie, maybe even the classic consoles, but Genesis and SNES don't fair so well from what Ive read. Maybe there will be room for improvement.
I shot some 240Hz video to estimate input lag, but I don't have the numbers yet.