If your current core isn't October 23rd nightly build or higher, then you're missing out. Even if you're on the latest Stable release (As of this post, that is August 1.9.0)
Update Retroarch to the latest nightly
(Not needed but a little sidenote) Delete your retroarch.cfg file if you don't have Online Updater on your retroarch menu yet
Convert your games to CHD format if you're playing with PS1 bin/cue CD rips. It will even save you some storage space. HOWEVER! Don't mess with your game if you really don't wanna lose your save tho. You are missing out a good performance boost, so if you ask me, it's worth replaying from scratch. Unless if someone here knows a workaround.
Keep the 3D Slider OFF if you're not using a 2DS
Settings\Video\Synchronization\Vertical Sync (Vsync) = OFF for 3D Games; ON for 2D Games
Settings\Audio\Output = dsp_thread
Quick Menu\Options\Advance GPU UNAI/PCSX4All Settings = ON
Close Content after this and reboot your game
Quick Menu\Options\Threaded Rendering = async
Quick Menu\Options\High-Res Downscaling
Restart so CD Access Method shows up
Quick Menu\Options\CD Access Method = Asynchronous