maybe its just me...
I'm running three types of tire sthis winter. Studs for ice, knobbies for snow/some ice general junk conditions and touring tires for the nice bits like this past week here in Colorado.
studs~ Slow but essential for commuting in ice/blackice. Sloooow rolling compared to the other two tyes. Depending on conditions, you will likely not run these very often in a year. Noisy! Count on peds turning to see whats making that buzzing noise behind them.
knobbies~ not as noisy. Better speed, better for most winter conditions. Harder to work than slicks but the bonus? You get a better work out and speed! when you go to slicks...A good 'workout' tire.
Slicks~ mmmMMM!! smoooooth and fast! I LOVE going to slicks! Even better if you find some high pressure ones! The easiest to work unless you get your cadence/distance up. Good re-motivation if your lacking any after too long on knobbies...
as for speed, I started the winter doing 13-17 on my commute. With slicks this week I was around 20 AND it was easier than this past fall when I started up on bikes again...
As stated by others, speed isn't important but as you get healthier, you can't help but GET faster!