This is one of the games included out-of-the-box, but Sony hard-coded it to run at 50Hz by default, despite being the NTSC-U version. Sometimes, there might be a strange blurring effect in cut-scenes. Thanks to Reddit user BL4Z3D247, (and the anonymous website submission) here is what needs to be done to fix it: 

In short, open the PS Classic’s internal storage with an FTP program, (here’s how) rename all of the MGS files and cuesheet/database addresses to “SCUS-xxxxx” rather than “SLUS-xxxxx”, then set the clock speed to 85% and the GPU to PEOPS.

Game runs well in AutoBleem, both in Standard and Enhanced resolution.

In RetroBoot cores, make sure to turn off Enhanced Resolution, or else the game will run slow.

(Bonus tip: For the “memory reading” easter egg, use AutoBleem and configure MGS and Castlevania: SOTN to share the same memory card.)