Compiling a subset of Ruby to C wouldn't be that hard, but making it compile to C that is fast enough to be worth it is. Not because the Ruby VM is particularly fast, but because the "naive" way of compiling Ruby to C still incurs almost all of the overhead.

E.g. TruffleRuby is fast in part because it will do things like try to avoid method calls for built in types where the standard operations haven't been overridden, but that requires a lot of extra machinery...

So I'm not sure how much compiling to C would help for gems that use C to speed things up.

I think maybe an easier target would be to compile C to a slightly augmented Ruby bytecode. If you control the C compiler you could do things like make C code follow the Ruby calling convention until/unless calling external C code, and avoid a lot of stack overhead.