Welcome to Fallout. Both Fallout 3 and New Vegas did the exact same thing for me. The more stuff you acquire, the more perks you get, the more quests you complete, the more the games starts bugging and running terribly. That's just part of it I guess. I'm also playing on a laptop. But, this laptop is no slouch. It plays most other PC games at max settings at 1080p. It also played Fallout 3 and New Vegas at high settings at 720p until I started getting into the game. 100 hours into the game and I've had to dumb down my graphics considerably. I'm hoping that if I had a really high end desktop this wouldn't be a problem as I hope to get one soon. All i can tell you is to kelp turning those graphics down or buy it for a console. Preferably the 360 since it appears to be getting exclusive DLC.
To prove my point, try starting out a new character and you'll see that it runs fine again. Play 20-30 hours with said character and you'll notice the same slowdown. A small price to pay for such a great game. Sigh