While I wouldn't be surpised that OCaml is fast, I'd like to be sure your disk cache was warm in both cases. I'd test it myself, but I don't have OCaml setup on my work computer.
I setup it on my computer, used it to count the lines of 250 files, it is indeed faster than my system's (ubuntu 18.04) wc, but it doesn't compute the total. I don't think this should change the timings much though.
A best of five run of the system's wc takes 0.042s, vs 0.028s for the OCaml version.
A best of five run of the system's wc takes 0.042s, vs 0.028s for the OCaml version.