It's not. It used to be, but it's not now, with Tcl interpreters after 8.0. [Refer to objective evidence.] It's necessary to answer the question, though, because so many antique claims to that effect continue to circulate.
Tcl is often somewhat slower than Perl for comparable operations. Algorithms that match Perl's regular expressions well generally favor Perl, sometimes by a large factor; a lot of work has gone into optimizing Perl's REs.
[Explain other penalties--correct Unicode and thread handling, ...]
Naive Tcl coders rarely understand the simple steps that boost Tcl performance. [Explain why this affects Tcl differentially.]
See http://www.equi4.com/md5/ for one example where Tcl is not that much slower.
Perhaps JCW will update these tests to reflect the new md5 code...