For me, about 3 mph. Of course all the other variables that affect winter riding speed, like air density, poor aerodynamics from bulky clothes, seasonal affective disorder and so forth play a role in further reducing speed. All I know that when I take off the studs in spring and put the slicks back on, I feel like I'm FLYING!!
I'd have to say around 3mph (5kmph) speed reduction for me as well, all other factors being equal. With bulkier clothing & actually riding on ice/snow/slush my average speed was reduced by a total of 6mph(10kmph). I find my microfleece long underwear (sorry, I refuse to call them tights) slow me down quite a bit.
Just mounting the front Nokian seems to produce zero drag increase, since, with my bike, most of the weight is on the rear wheel. Adding the rear Nokian does produce a drag increase that I notice -- particularly in the spring when they come off. Exactly what the speed decrease might be is uncertain, since I have a lot of stoplights on my route. Last March, I gained 5 minutes on a 30 minute ride first trip without them. That might have been just the excitement of springtime plus the luck of the lights.
Commutes can be a lot faster over ice and snow with them, of course -- both from being able to ride over shorter, but less-clear road segnents and from higher speeds.
Hard to quantify, but probably only a mile per hour or two on my routes.