That's a problem with some studios, is that they get anxious for the movie to be finished and released that it becomes sloppy and rushed: on average, when you're working on a typical movie, you're looking at a good year and a half to two years' worth of work, and that's just for the actual production, filming, post-production, editing, etc. So if Disney's wanting a summer release for the sequel, we're looking at possible problems like rushed/sloppy story telling, plot holes, etc.
We should be glad this isn't an animated movie, because those can sometimes take upward of five years to finish.