Cold generally has a negative impact on running performance.
When your body temperature falls (a) your cardiac output decreases meaning there is less oxygen being pumped around your body; (b) oxygen transfer (e.g. to muscle cells)is less effective; and (c) (I think) you're having to use a proportion of energy to maintain your core body temperature.
All that basically means is that (like for like) any single action requires more effort in the cold than in normal temperatures. I guess, as you warm up it should improve vs. when you first start but over a fixed distance it should be harder and even when you're "warmed up" there is still a proportion of energy being used, for example, to keep your extremeties warm!
If you have asthma, I imagine it's even harder as you're even shorter on oxygen. Also, I suspect you take shorter, harder breaths when it's really cold which probably doesn't help (well, I know I breathe harder in the cold).
That being said, I had an 8 mile run this morning and it was actually absolutely fine, but then, 'tis the south of the country. Did stumble a couple of times though with colder, stiffer feet....
I actually don't mind the cold but struggle when it gets too hot though don't think I'll have to be worry about that just yet! Did a half marathon in September this year when the temperature was in the mid 20s and was incredibly hard work - temperature makes a real difference to your performance.