Lag on CRTV with RF Modulator?

Just got a PS3, tried out Splatterhouse. I set it up on my CRTV through an RF Modulator.

I noticed the game would slow down mostly when getting trophy’s. Is this a problem with Splatterhouse, the RF modulator, or my TV?

It sounds like the game is to blame. If it was the RF unit it would be slow all the time, and the same with the tv - neither device would be doing different when you’re getting trophies.
I don’t know this for sure, but my best guess is that the ps3 tries to register your trophy to the web as you collect it, and this could slow the console down?

I agree, this is the fault of your game. Even if the Console do not try to auto-update as you unlock trophies, this is a problem with the game’s code and not your hardware.