I switched off Wi-Fi and bumped my 920 down to 2G/EDGE speeds.
I am on the AT&T network in the metro DC area ish.
I found it takes much longer, simply saying "hello" takes ~16 seconds for a "hi" back.
And voice recognition is much slower, going noticeably word by word., I suspect a significant dip in accuracy too.
Bumping it up to 3G/3.5G, (ATT's H, H+ network), a response to "hello" flew back to me in only 2-4 seconds.
However, writing out your commands, as opposed to speaking it, cut the time to respond to my typed out "hello" to ~6 seconds on 2G/EDGE.
So if I were on 2G, I wouldn't be talking much.
2G/EDGE Speedtest.net results: very-high ms ping / 0.00 Mbps down / 0.00 Mbps up
Microsoft Speed Test results: seems the same, I suspect they timed out.
Results on 3G/H/3.5G/fauxG/H+ (couldn't choose and it switched between):
130 ms ping / 2.32 Mbps down / 1.62 Mbps up