Because that was all the processor was doing. No virtual memory, no disk, no IO, no graphics. Just feeding the DSP data on one side and NAT routing on the other.
Under the same line conditions, the PII running windows was useless running at 450Mhz. Mouse would barely respond, keyboard was lagging, screen wouldn't update.
Like you said, depending on what you are trying to achieve, you can perform near miracles on confined hardware if you have confined demands.