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.



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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.




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[[Explain other penalties--correct Unicode and thread handling, ...]]



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Naive Tcl coders rarely understand the simple steps that boost [Tcl performance].  [[Explain
why this affects Tcl differentially.]]

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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...

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[Arjen Markus] From my own experience I can add that Perl and Tcl may have different ideas about what seems the same task. For instance: reading a file line by line. I noticed in a slightly naive test that Tcl would convert the end-of-line sequence, and Perl would not. (Note: I have not delved into the details and it may be specific to the particular releases I used then). This means that setting up a performance test requires a thorough understanding of what each language will do.

Furthermore, slight changes in the implementation of the algorithms may make all the difference. (Somewhere I see apples and pears lurking behind shrubberies of intricacies!)