0. vfork (which is sometimes better than CoW fork) + execve if exec is the only outcome of the spawned child. Or, use posix_spawn where available.

1. Inner-loop hot path code {sh,c}ould be made a bash built-in after proving that it's the source of a real performance bottleneck. (Just say "no" to premature optimization.) Otherwise, rewrite the whole thing in something performant enough like C, C++, Rust, etc.