PDQ: Pretty Damn Quick is a tool for analyzing computer systems performance. It has been the basis for a couple of books by Neil Gunther, the latest of which is "Analyzing Computer Systems Performance with Perl::PDQ ".
As the name suggests, the book uses Perl. But since all the author did was use SWIG to wrap his PDQ C library, you can pretty easily substitute Tcl for Perl (which is what I am in the process of doing -- Todd Coram).
Interestingly, Tcl with PDQ runs a tad bit faster than Perl::PDQ (probably due to a faster Tcl startup).