I've written numerous realtime terrain rendering engines, so this is a familiar topic.
Familiar enough to know where most time will be spent?
If so then maybe you can write code for just that part in different languages and compare.
But according to the The Computer Language Benchmarks Game Haskell often beats OCaml, if only by rather small fractions — there remains the problem, that this benchmark takes only very specific samples.
The benchmarks game reports 4 sets of results - one core and quad core, 32 or 64 bit Ubuntu - and you may find that the OCaml or Haskell benchmark programs perform better or worse depending on the platform.
All a benchmark can do is take very specific samples, and of course you should disregard comparisons on tasks that are unlike where most time will be spent in your application - large integer arithmetic? regex? strings? - and look at the comparisons that are most like what you intend to do.
