On my system, I get a different result:

               Rate   split_pos  grep_split  substr_map substr_loop      unpack
split_pos    4596/s          --        -53%        -71%        -79%        -82%
grep_split   9843/s        114%          --        -37%        -54%        -61%
substr_map  15674/s        241%         59%          --        -27%        -38%
substr_loop 21459/s        367%        118%         37%          --        -15%
unpack      25381/s        452%        158%         62%         18%          --

Your performance characteristics depend on all sorts of things relating to your CPU, its cache, bus speed, memory, etc.

But as far as ways to make it faster, you might want to use an idiomatic for loop instead of the C-style loop.