Ways to improve Shader Loading

> Is there anyway I can have RPCS3 preload the shaders used in Persona 5?
It preloads the shaders you already found on every boot. You can't preload shaders you didn't find on game code yet, of course.

> What can I do to increase the fps?
Post a log file to check settings

> Is there some way I can tone down the amount of npc's the game spawns that way the game doesn't need to load as many shaders?
Not without modifying the game itself, I'm not aware of any mods that do that

Shaders are already preloaded on first boot, the ones you found on previous gameplay of course. Shader cache has been implemented a long time ago, that predates the async shader compilation post you linked.