I’ve tried RetroArch 1.7.2 and runahead is quite magical how it removes input latency. I tried with MegaDrive and removed one frame of lag in my test game.
Currently your hardware needs to be powerful enough to run two of your chosen core. On low power hardware only a few less demanding emulators will be usable with this feature.
And you need to manually check how many frames of lag the game you’re playing has. This is easy: pause, hold an action button for the rest of the process, press K to advance a frame at a time until you see the action happen. Each press of K is a frame of lag.
So, now that this is out, the RetroArch team are optimising emulators to squeeze out some extra performance gains, and they plan to improve the runahead system to lower performance requirements.
Bonus is that even if you never use runahead you’ll benefit from the performance gains that are coming to emulators.