It depends not only on the POS itself (old models vs. new ones) but on the kind of connection.

Here in Italy, besides "portable" POS that have a SIM card and go through GSM/GPRS (and are "good speed, but not that much fast") now also 3g/4g, the "corded version" can be:

1) Dial in (analogic)

2) Dial in (ISDN)

3) IP connected

The difference between #2 and #1 is like 4/5 times faster ISDN vs. analogic, and the IP (provided that there are no network issues) is instantaneous.

I would say:

1) 5-20 seconds

2) 1-5 seconds (and GPRS is roughly the same, 3g/4g is on the lower side)

3) 0-1 seconds (really, the sheer moment you press the green button, the receipt starts being printed)