[docs] [issue29724] Itertools docs propose a harmful “speedup” without any explanation

[Raymond Hettinger]

Raymond Hettinger added the comment:

Additional note:  For folks interested in performance, it is common for people not to realize that function call overhead can dominate their timings (see this recent SO question as a recent, but typical example http://stackoverflow.com/questions/41772054 ).

In any case, the note needs to remain because people commonly move these recipes into production code and need to be warned that basic optimizations were skipped.  This is important in a module where improved performance is one of the primary goals.

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