I used to benchmark using fibonacci, but the recursive method is just awful because it does a lot of function calls and you're essentially benchmarking that. Then I switched to finding primes using the Sieve of Sundaram. It uses arrays, hashes/maps/dicts and two loops. Also wastes a lot of memory if you don't split your search domain into ranges. The surprise was that Go and D (in that order) turned out to be faster than Rust mainly due to Rust's HashMap SipHash algorithm. I gave up trying to use other hash libraries (SeaHash specifically) that are not part of the standard lib, because it was quite frustrating compared to D.