> I had just heard of Rust and rewrote it in Rust, basically transforming F# to Rust syntax and trying to eliminate as many copies as possible. Now the whole execution took 8s.

For what it is worth, I've had a very similar experience with the re-write of an internal tool in Rust.

I was in pure disbelief, running the next command in the toolchain this tool runs in, expecting it to fail on some empty input files but it worked. 20x speed-ups almost feel like some essential work must have been missed or skipped but when you realize that's not the case, it is a different kind of pleasure.