Whatever language your coworker used.
(I could tell you that any macro assembler will let you write code that would rip through your data, but seriously, are you going to spend months writing assembly just to save a few seconds of CPU time? Rewriting a program is fun but it's not practical.)
Whip out your profiler, point it at your horribly performing log parser, and fix the performance problems. If it's a common language, there will be people here who can help.
