Author of pycapnp here. For those too lazy to click through, pycapnp is tremendously slower than C++/Rust. The equivalent benchmarks take about 20-50x as long.

What's more interesting to me is comparing it to other Python serialization libraries. I've been meaning to update the benchmarks with some popular ones (msgpack, thrift, and the built-in json module are at the top of my list). If there's some interest, I'll go ahead and post it to HN when I'm done.