[timrobinson]

Because PS/2 is interrupt driven whereas USB relies on the CPU polling for events.

https://en.wikipedia.org/wiki/PS/2_port#Legacy_port_status_a...

[ahartmetz]

It's mostly irrelevant. USB uses polling internally, but the polling frequency is high and an interrupt is generated on the bus side when something has happened. I have taken latency measurements on PC hardware for a serial communication situation, and USB latency was a very small part of the problem. 1-2 milliseconds typically.

[kevin_thibedeau]

It isn't irrelevant because USB packets still have to wend their way through the host stack after reception. Interrupt driven IO is always lowest latency.

[unwind]

There is no separate interrupt line that devices can use to interrupt the host. All USB traffic is started by the host.

[ahartmetz]

Yes, but with sufficiently high polling frequency it doesn't matter very much. (It is true that each hub may add latency, as your sibling comment noted.) Regarding the "interrupt on the bus side", I may have expressed myself badly - I meant PCI(e) bus.