When it starts to lag, save and turn off your console. Then turn it back on, this will clear the cache. the do this: -Settings-Game Settings-PS Upscaler (OFF) PS Smoothing (OFF)
-Settings-System Settings-Display [What's New] (OFF) Notification Messages (DO NOT DISPLAY)
-Settings-Display Settings-Cross Color Reduction Filter (OFF) RGB Full Range (Limited) Y Pb/Cb Pr/Cr Super-White (OFF) Deep Color Output? (OFF). After that is done start game and turn off autosaves. Helps big time.
You do realize that PS Smoothing and Upscaler are ONLY for PS1 and 2 classics, when using an HDTV right? They have nothing to do with a regular game disc like Rimlag or any other non PS/PS2 game.
Turning off Display 'What's New' can actually help, but the difference is negligible, you'd be better off just disabling your network connection.
Not sure about your last suggestion, since I've never actually messed with that one.