Not sure about MacOS plus it’s not REPL (sorry if that is key), but have you tried faster compilers for prototyping? Such as TCC (Bellard’s Tiny C Compiler) as long as performance isn’t critical and if you don’t need multi-threading.

Clang is also pretty good (for a simple benchmark, the result I got was that Clang with opt:size gives similar performance to plain GCC but with half the compile time). (nlvm is also a thing but I have no idea how that’d compare even to Nim compiling code via Clang)

With some setup, maybe some form of hot code reloading?