You don't need C's performance in 99% of the code. In fact, in most projects you far from need C's performance anywhere. Had it been the case, then fewer would write Ruby, Python or PHP these days.

The performance of OCaml is really good if you write no imperative code. If you have a cost-centre that bothers you, it is pretty easy to get it fast: OCamls compiler does very little optimization, so a bit of manual rewriting helps a long way. There are several documents on the net that explains how to manually optimize a cost centre.