[snarfy]

There are a variety of factors that add up to many ms of lag. USB and HDMI are a few that come to mind. USB is incredibly complex when compared to e.g. ps/2. If you are building an embedded board like rasberry pi and you want to add ps/2, you just add a couple pins. If you want to add usb, you buy an extra chip.

[duped]

Many CPUs have USB support. Pin count also doesn't correlate to latency, what matters is the minimum buffer size and transfer speed.

USB can beat PS/2 for latency without issue.

[WithinReason]

Isn't PS/2 interrupt based, while USB is based on periodic polling?

[duped]

Polling is often lower latency than interrupts on modern hardware, since it can run at much finer grains and has so much less overhead to startup over an ISR.

[wtallis]

That's often true on a scale of microseconds, but USB input devices are polled at intervals on millisecond scale.

[cma]

Only in the slowest modes. It is possible to go faster than the 1ms poll rate, though it is back to interrupts:

> Transaction latency

> For low speed (1.5 Mbit/s) and full speed (12 Mbit/s) devices the shortest time for a transaction in one direction is 1 ms.[6] High speed (480 Mbit/s) uses transactions within each micro frame (125 µs)[7] where using 1-byte interrupt packet results in a minimal response time of 940 ns. 4-byte interrupt packet results in 984 ns.

https://www.wikipedia.org/wiki/USB_(Communications)

[bschwindHN]

Ben Eater has a good video on this:

https://youtu.be/wdgULBpRoXk?t=1765

[hffftz]

> If you want to add usb, you buy an extra chip.

Do you know how many chips are in a cable modem/router combo?

[formerly_proven]

1 or 2, depends on the kind of modem. VDSL2+ modems might have three, though one of those is the line driver which is zero-delay for the purposes of a computer.

[hffftz]

I've seen many more chips last time I opened one....

[jaywalk]

Most of those chips have zero impact on latency, and many aren't even involved in the actual packet flow.

[ben-schaaf]

Modems/routers don't use polling nor do they need debouncing. The default USB polling rate is 125Hz, which is already 8ms of latency. Debouncing depends on the switch and is generally a couple milliseconds as well.