rb_enc_from_index(index) doesn't need locking if index specify
ASCII/US_ASCCII/UTF-8.
rb_enc_from_index() is called frequently so it has impact.

                               user     system      total        real
r_parallel/miniruby       174  0.000209   0.000000   5.559872 (  1.811501)
r_parallel/master_mini    175  0.000238   0.000000  12.664707 (  3.523641)

(repeat x1000 `s.split(/,/)` where s = '0,,' * 1000)