If you turned off the rendering of the bridge, it wouldn't really be much faster. The speed of the physics calculations is by far the limiting factor of your performance.
Am I alone in hating the levels with asymmetric anchors? I don't so much mind the ones with anchors only on one side of the water. It's the ones with only one anchor placed on either side of the track that really piss me off.
On those bridges, I generally just make a bridge that can support the train on 'easy', regardless if its clean or not, then move on.
With the current system of complex/3D building, it's NOT FUN to try to balance out the load of the bridge from side to side.
Specifically, the levels that really agitate me are:
Complex 5
Map pack 4, 6, and 9 (!).
Thats what I figured

. Oh well, it doesn't bother me enough to taint my love for this game

.
It's a challenge, I like it.
I like them too, the levels that bother me are the really long ones, as they take about 10 - 20 minutes to run through on my machine. Would having an option to stop rendering the bridge help with this? That way if you knew your bridge was strong enough, you could let it just concentrate on the physics calculations. Probably not an issue for most people, but on my k6-2 350, those levels can take awhile. ;)