Yep, no excuse for the algorithm I'm using, it's just strange since (locally) the factor between release and debug is sometimes more than hundred fold.

Thanks, will ask in the forum.

Why are Rust (and probably others) are compiled and executed in debug mode?
/workspace/rust/target/debug/deps/challenge

This makes it quite painful to complete this challenge in < 12_000 ms as the compiler can't optimize.

Is it possible to actually change the execution to release mode?