How much slower should I expect to be in winter rides? I typically do ~100km rides (almost always alone). In summer, I manage average speeds of 30km/h or more. This is the first (of only three in total anyhow) winter in which I keep riding somewhat regularly in winter.
I did &gt; 1000 km per month in the summer months, and around 700km (some of which indoors) in the winter months (but extra running etc.). So I would expect my form and fitness to be somewhat lower, but not dramatically so. For instance, average watts on the indoor trainer as well as times when going running are fairly constant. (My PM also shows fairly constant wattage on the bike.)
Yet, average speeds are more like 27-28 km/h at the moment.
(In my case, although I hope the question could be of more general interest, I also ride a new bike which I had hoped would be faster although it is an endurance bike, simply because it is more high level in terms of the frame than my previous fairly entry level alu one.)
My question: Is that an expected decrease in speed? If so, is it possible to break down this difference into different components (next to, possibly, less fitness)? I think of

typically more windy conditions, which hurts average speed
more layers of and less aerodynamic clothes
wider tyres (28mm insted of 25mm for me, also lower quality ones; also, the endurance bike maybe has somewhat less agressive geometry)
more rolling resistance on wetter surfaces
more careful riding style on wet surfaces (surely hard to quantify, but definitely applies to me)
I heard something about colder air implying more resistance
more energy needed to keep the body warm (although I rather overdress, I think)
psychology?
anything else?
This source goes into quite a bite of detail on the subject.
The three main reasons they go into are higher air resistance in the cold because cold air is more dense.  Increased rolling resistance as cold tires cannot deform as much as warm ones. They also mention the bulkier clothing causing more air resistance.
One advantage of colder weather is that you are less likely to overheat. This can increase performance.  I have felt this effect of being able to maintain my efforts longer on cool fall days, while cycling on my trainer is often more difficult as there is no air flow to carry the heat away.  Also, cycling on hot summer days can also be exhausting.
Just be careful not to overdress for the cold as having too many layers could also  make you overheat. It's ok to start off cold as you will warm up during the ride.