For convenience in my personal code I has switched to using a comment proc. I can't tell you if it has a performance cost but it is very convenient for commenting out long blocks, providing all your "{" have matching "}" within the commented block.
proc comment {args} {}
comment {
lots of commented out stuff
in here {[someproc]}
}
It doesn't look to me like the code in brackets in comments is executed in tclsh. Is there something different going on here?