While I can't comment on performance hits that adding comments will do, I can add some "wisdom" to the subject of adding comments. I have a pretty basic rule:

Comment for yourself.

Most of the hack/program/whatever you just wrote will be erased from "short term" memory soon. In 6 months (a round figure, but pretty good) you will have FORGOTTEN that "special" think you now know how it works. Write the comments so YOU, the programmer, can figure it out again. Trust me! You will forget the quirks, and spend LOTS of time scratching your head.

A simple rule, and has served me well. Sometimes paragraphs are needed, sometimes a simple note. Make it so you can understand.

An exercise: look back on some of your OWN old code and see if you still understand it.