Thanks, I finally understood what you are saying. Indeed, the code uses iswspace to test all characters, wide or normal. Strange design choice. For whatever it's worth, even just changing
if (iswspace(wch))
to something like
if (domulti && iswspace(wch))
...
else if (!domulti && isspace(wch))
...
got something like a 10% speedup on my machine. And replacing isspace with an explicit condition like yours is much faster still. I checked, isspace is macro-expanded to a table lookup and a mask, but apparently that's still slower than your explicit check. I'm a bit surprised by this but won't investigate further at the moment.
It has the worst of both worlds: it incorrectly counts the number of words when there is non-ASCII whitespace (since mbrtowc is not used), but it pays the penalty of using iswspace. It's also not in correspondence with POSIX, which states:
The wc utility shall consider a word to be a non-zero-length string of characters delimited by white space.
[...]
C_CTYPE
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments and input files) and which characters are defined as white space characters.
