O'caml and Haskell are extremely niche in the web server world, and are not especially faster than go. C# is not blazingly fast either. They all seem to be in the same ballpark.
I've been paying a lot of attention and, in my experience (but also according to the highly biased benchmark game), C#/Java/OCaml/Haskell/Go are in the same league, i.e usually 3-10 times slower than C/C++/Ada/Rust/D, and 3-10 times faster than "scripting" languages like Python/PHP/Ruby/Perl.
With (in my experience) go being a little faster than java and haskell, and o'caml being slightly faster than anyone, but nothing significative enough for me to expect an article titled "my Java webapp was too slow so I rewrote it in O'Caml" anytime soon.
Benchmarking is very relative, though. I'd be glad to read evidence showing go is significantly slower than any of those in some domain.