> The situation we're talking about is: foo.h needs sys/types.h to work
> properly, and if foo.h should include sys/types.h, or if the
> application pulls it in.  Where is the performance difference?

When there's also bar.h which needs (and putatively includes)
sys/types.h.  The .c-includes paradigm reads sys/types.h only once; the
.h-includes paradigm reads sys/types.h once per file that uses it.

Except that, as has been pointed out multiple times by now, gcc
optimizes this away if sys/types.h is protected with the canonical
ifdef/define/endif bracket.  There still will be a performance hit, but
I fully expect that it will be too small to matter, quite possibly too
small to measure.  (Anyone care enough to bother finding out?)

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