How much slower should I expect to be in winter rides?
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.