COD4 Multiplayer FPS vs. Singleplayer FPS?

I have been really frustrated lately about my copy of COD4's performance (NO BOOTCAMP) on my MacBook Pro 13" 2.53 GHz. It has the 9400m card in it (which is decent...) and lately the SP version has been running at maxed out settings with about 10-15 FPS on average, while in MP the same settings will be completely maxed and it'll be running at about 50-60 FPS. Why the incredible difference?! I know that I am pushing the limits with the SP settings because the game has to process the AI and stuff, but with these specs shouldn't I be getting more than that?

Model Name:​

MacBook Pro

Model Identifier:​

MacBookPro5,5

Processor Name:​

Intel Core 2 Duo

Processor Speed:​

2.53 GHz

Number Of Processors:​

1

Total Number Of Cores:​

2

L2 Cache:​

3 MB

Memory:​

4 GB

Bus Speed:​

1.07 GHz

I can completely understand having to process AI, but there must be SOME hidden process for there to be a 30 FPS difference!
(Yes, I have the Mac version)