For those of you who use emulators on your Rpi and experience input lag from controllers, I think Ive found the best settings to eliminate input lag. Compared to the actual consoles, these settings virtually eliminated input lag on a Logitech dual shock, SNES replica, and NES replica.
-Raspberry Pi overclock set to 800mhz
-Edit the retroarch.cfg file found at /opt/retropie/configs/all and change:
video_hard_sync = True
video_frame_delay = 10
Those small changes allowed me to have virtually input lag free gaming on all emulators. Your settings may need additional tuning depending on the response time/model of your TV (mine is a Samsung LCD with Game Mode which goes to a lower response time for gaming) so you may want to play with the settings a bit if these arent perfect.
It also may be worth noting that Im using a Raspberry Pi 2. Older models may not respond the same. I found under or overclocking beyond 800mhz threw all video settings off completely and made input lag worse. Id try these settings as a starting point and tweak if necessary.
Just wanted to share, it took a lot of testing to get everything dialed in so Im hoping I can save someone a buttload of time.