It depends what the consumer is doing with the data as it exits the buffer. If it’s a terminal program printing every character, then it’s going to be slow. Or more generally if it’s any program that doesn’t have its own buffering, then it will become the bottleneck so the slowdown will depend on how it processes input.

Ultimately even “no buffer” still has a buffer, which is the number of bits it reads at a time. Maybe that’s 1, or 64, but it still needs some boundary between iterations.