Slow times are all relative now at WDW. Through pricing changes and scheduling of events, WDW has done a good job of distributing the crowd throughout the year. Instead of slow vs busy its usually now more busy vs slammed. Yes, anytime when kids are in school will tend to be a less busy than during vacation periods, and other than Marathon Weekend in early Jan, there aren't popular events booked like the holiday events or F&W.
Yes, during those non-peak periods, park hours are usually shorter. For instance, I think normal close for most nights in Jan is 9 or earlier right now. But, since the crowds are lighter (relatively) you can still accomplish roughly the same amount as you can when the park is open later during busy times.
Jan/Feb can still be cold. Or it can be hot. I usually go down for Marathon Weekend (usually first weekend after NYE) and I've had it be 30 degrees and sleeting, and I've had it be 80. The biggest consistent is that the mornings/nights tend to be cooler than the middle of the day, so even when it hit 80 in the afternoon, it was still low 50s to start/end the day.