edit: weirder, naive Julia (1.7 on Linux) is terrible. 45 KB/s.

Ruby is slightly slower than Python or Rust, 11 MB/s. I was expecting Python to be slowest among the interpreted languages, apparently not. Pypy yields a speedup to 70 MB/s.

Summing naive implementations (for each i: start with empty string, append fizz/buzz based on the modulus of each, then conditionally append i and print):

Julia 45 KB/s Ruby 11 MB/s CPython 15 MB/s Rust 16 MB/s Pypy 70 MB/s C 160 MB/s