Comments will never slow your code down. When your script is run, comments are simply thrown out, since they have no impact on the actual workings of the program.
The only disadvantage to having a huge amount of comments is that you may be causing confusion. Try to be concise and clear, but use as many words as you must to get your point across.
Bad comments are ones that are completely obvious simply from looking at the code. For example,
# Print the contents of the string variable $foo
That comment would be worthless, since even a non-programmer can just glance at the code and tell what it does.