Lets examine each of these claims and attempt to weigh each as a factor in determining winter speed differences. In some cases the data is not readily at hand, and we will have to use intuitive measures. We will assign and estimate of overall effect to each factor. If a factor is correctable, it gets the weight of its corrected state. 1 Cold Bearings.
Cold bearings are indeed slightly stiffer to turn.
Too heavy of a grease can cause problems in your freewheel when you want to coast, the chain wants to keep turning the cranks. You might also find your cranks difficult to turn in really cold temperatures. But this is, in the end, a maintenance problem, like a dry chain, or a rubbing brake. It can be fixed by the application of the proper grease. Time it till it stops.