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.

Sorry but it's not a problem of the kata; maybe a CW problem. You can report an issue at CW. Look at the left panel where you can find "Bug Reports" in "Forum".
Note at the top of the page: that didn't prevent 268 guys to pass the Rust kata. My reference solution takes about 1500 ms.

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?