Depending on the effects you want to do there could be many approaches, mix of match of new and old tech.
If you want a true ps1 graphics on your whole game, there is some better ways to do it, which will ofcourse improve performance dramatically so you can take advantage of those approaches.
Like
Removing anti alising, lowering render resolution, resolution and sampling for textures, forward shading, lights etc. in project settings overall so boost your performance while achieving a better retro look. Here is a video tutorial to those aspects of the rendering.