PC Games Hardware has benchmarked the recently released demo for Left 4 Dead 2 compared with the version of the demo released in Germany that has some of the gore removed, and found the cut version runs about 40%(!) faster than the uncut version. Word is:

The direct comparison between cut an uncut version reveals huge differences in performance. It doesn't matter if we use a dual-core E6600 or quad-core Q6600 processor, the less violent version of the demo runs about 40 percent faster that the uncut version. Furthermore the difference between the two uncut results (dual-core vs. quad-core) is bigger, too, what indicates a higher CPU workload.

Since it is currently possible to play on "uncut" servers with a cut version of Left 4 Dead 2, the question arises if this could be an unfair advantage for some players. A scene that lags in the uncut version might be running smoothly in the cut version - especially in Versus Mode this could make the difference between death and survival. It is of course possible to lower the amount of violence in the uncut version via the command console though.