Game lags in Steam

[Labyrinthine]

When I test play my game in Steam, the framerate drops to 4-5 frames per second at random times, usually after I've played some time. This does not happen when I test play the game outside of Steam. I used an achievement script in the game earlier, but removed it and the achs but the game still lags.
I don't know if this is the right address to ask about this, but I'd be grateful if someone had any idea what's going on.

 

[Hudell]

Are you encrypting your images and audio? Have you tried playing the game with encrypted files but without steam to check if the same problem occurs?

 

[Labyrinthine]

I have only been using the Vx Ace built-in encryption where you have to check the boxes "encrypt game data" and "include RTP." I don't think it encrypts images or audio.