Dunno on PC but on 360 it's an almost locked 60fps, with Digital Foundry finding only a "handful" of dropped frames across all their tests.
For all my gripes with the game, framerate is the one thing it got undeniably right.
Screen tear was also an issue at the time:
"it's clear that the additional load on GPU fill-rate caused by this liberal application of full res alpha can prove to be too much - we see a momentary drop in performance plus screen-tear."
"The effects are short-lived however and don't tend to impact on your enjoyment of the game at all - the screen-tear is definitely more noticeable than the frame-rate drop, but you get the idea this was all factored in and assessed by id. There's a feeling that its load balancing tech isn't quite so deeply applied in the driving sections as it is in the shooting - and it's almost certain that rendering the Wasteland offers far more of a challenge to the engine than the enclosed corridor shooting of most of the FPS missions."