You may go a little faster with `--opt-level=3` and no `-Z debug-info` for the Rust.

That would be quite useful. Otherwise it's very hard to tease apart the differences between optimization back ends of GCC and LLVM vs the Rust and C++ front ends. I feel like I have seen benchmarks showing differences around 5-20% between LLVM and GCC in speed, so the it will have an effect.

Update: the compilation error was actually due to some missing includes. https://github.com/kentonv/capnproto/pull/47

Switching compilers on the C++ side doesn't seem to make a significant difference in performance.