Function call overhead in cgo is considerable. You're right that it's not from copying, but the runtime scheduler still has to coordinate the blocking call, and the stack needs to be switched out to the C stack, kind of like a context switch.
True, just benchmarked it and found the function call overhead to be about 1.85834729e-7 seconds (185 ns). Which isn't much, but the pure C version would obviously be single nanoseconds for the handful of instructions needed depending on the function call.
