I wonder how much this statement still holds. I've never used the OCaml bootstrap compiler but performance wise, the rust compiler has improved incredibly since the 1.0 release.
An apple to apple comparison is impossible because rustboot compiled a very different language. But I suspect suitably updated rustboot would be still faster because compilation time is dominated by LLVM.
Rustboot's code generator was generally slower than LLVM. I think in some small test cases it might have been faster, but when implementing stuff like structure copies rustboot's codegen was horrendously slow because it would overload the register allocator.