This has not been true since the introduction of the bytecode compiler in Tcl 8.4 (well over ten years ago now). Tcl 8.4+ will byte code compile the proc's, an

This has not been true since the introduction of the bytecode compiler in Tcl 8.4 (well over ten years ago now). Tcl 8.4+ will byte code compile the proc's, and part of that is removing the comments from the code execution path. So comments only get 'seen' once (at byte code compilation time, upon first use of a proc) and then have no effect on execution performance from that point forward.