I changed the strings on my cello about a week ago, and it seems that these new strings just aren't as "slick" as the old ones. It feels as if they stick to my fingers, and I have to fight to shift properly in and out of thumb position. These strings really do sound great, and they have a very nice resonance, but it is noticeably harder to play in tune with them. Has anyone experienced this problem before? Is there some sort of break in period for new strings?
Hmmm, no I don't think I've noticed a particular break-in period myself, but maybe I just haven't noticed. If you've changed brands or makes at all, you might just have to get used to a new feel. I have however, had days when it's very hard to shift smoothly, often due to weather. One thing that has helped a great deal is to take an almond and break it in half. Rub the open side of one of the halves up and down the strings in the area of shifting (absolutely avoid any area that might be bowed). In fact, probably only rub the string up to about fourth position. Do it with only a few strokes per string to see if it helps, and only do more strokes if it hasn't fixed the problem as much as you want.
This trick has been suggested to me on several different occasions by professional cellists completely independently of each other. I've used it myself (although very infrequently, only on bad days when I have to perform) for over 20 years, and I haven't noticed any particular problems with string life as a result of doing it, but I can't say with certainty whether it might have some mild deleterious effects that I simply haven't noticed. I always wipe the strings down well after playing with a lint-free cloth, but we should probably try to always do that anyway.
If you haven't switched brands of the strings, it is possible that it may simply be a breaking-in period. Some types of strings take longer to break in than others. Some tricks that could speed up the process include: playing slow chromatic scales all the way up one string with your bow very close to the bridge; asking another cellist to play some difficult repertoire on it (especially forte passages); and the idea of @Pat Muchmore.