There is a small hit on speed, as the interpreter loads twice, but the hit is better than running without tainting.
I have to disagree in respect to the performance hit being better than running sans taint mode. If you as a programmer really know what you are doing in a public environment such as CGI, then taint mode is not that necessary. As long as you take all the proper precautions as you would while under taint mode, then you will be fine.
Now, someone will most likely counter this claim with the "well, what if you forget to check for something?" question. Quite true, but like I said, such a thing wouldn't likely occur to someone who knows what they are doing.