You're confusing throughput with latency. Those old systems were objectively more responsive because the keyboard and mouse were tied directly to high-priority IRQs which were serviced in the OS. Now we have USB keyboards and mice, which go on the same IRQ lines as every other USB device, and even if the OS could service them specially, it doesn't. So keyboard and mouse events go on a queue of things to do: disk accesses, network packets, etc. And the whole system feels slower because of the time lag between you sending a command to the machine via kb or mouse, and the machine actually receiving it.