This quote finds there are more issues than just the 5000.
" framerate between 20 to 50 sounds about right. The reduction over time is likely throttling from the MacBook Air's poor thermal dissipation. (You don't expect a very good cooling system to fit into a wafer-thin laptop, do you?)"
While it's a very nice computer, it's not a gamer.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic