I would like to share with you a way I discovered when upgrading to version 5.8 that allows for immense improvement in image quality in PSX games.
I always left the "soft image" mode activated to improve the graphics and I saw that it helps the best image for the emulators of Nes, Snes etc .... I also activated interesting shaders in MegaDrive emulators, Snes etc .... However, when I try to activate these shaders in the PSX emulator, the games start to very slowly and in no way improves the image, resulting in the impossibility of playing these games.
However, I discovered recently in this latest version 5.8, when accessing the menu of retroArch (Hotkey + B ) that, under options there are two interesting options: "Enable Interlacing ...", and "Enable Enhanced resolution (Slow)". I activated these options by changing from "disabled" to "enabled" and pressed "B" to save the settings and then, magic happens: The image is completely smooth, without pixelation, very similar to what the OpenGL plugin(.so file) does in the ePSXe emulator for Windows / Mac / Linux / Android.
With this configuration I believe that the best possible image is applied to this emulator and it certainly becomes much better to play PSX games ...
I do not know if this option already existed in previous versions, because I did not really watch it, but it really is very satisfying to be able to play Playstation 1 with such image quality.
As this is not an easy-to-see option for lay users like me, so I decided to share here for those who need it ... for me, this is the best possible configuration for this emulator.
Remember that: I'm using Rpi3 that has a better hardware configuration than the previous ones and since this option certainly consumes more features of your device, I believe that in previous versions of Raspberry, it could be slow as the mode itself already warns in its Name, and it is necessary to disable it if let is very slow ...