I have a Trek 7.2 FX with Schwalbe Winter Marathon tires. They are excellent on ice but lose all traction in >3" of snow.

I bike as the primary means of transportation, namely my work commute. My LBS had some Salsa fat bikes in the store but they told me the air pressure is very low in the tires. So that you can't go very fast (you said your top speed was only 20 MPH??!). I can go at least 35 MPH downhill on my Trek. I'd like to get one of these bikes for heavy snow days, but the top speed concerns me. Is that really true that the tire pressure restricts the top speed to around 20 MPH? And does the low air pressure also require a lot more effort to pedal the bike around?