I found that grep -Fw is extremely slow in spite of whether in
multibyte locales or not.

$ yes 'abcdefg hijklmn opqrstu vwxyz' | head -100000 >k
$ time -p env LC_ALL=C grep -Fw vwxy k
real 14.03
user 12.51
sys 0.74
$ time -p env LC_ALL=ja_JP.eucJP grep -Fw vwxy k
real 14.29
user 12.67
sys 0.50

$ time -p env LC_ALL=C grep -w vwxy k
real 0.11
user 0.01
sys 0.09
$ time -p env LC_ALL=ja_JP.eucJP grep -w vwxy k
real 0.89
user 0.71
sys 0.15

First patch fixes the problem.  Second patch changes as using grep
matcher for grep -Fw in single byte locales.

In single byte locales, DFA (not regex) is also used for words matching,
and it is very fast as above result.