ST Disney's Interest in Episode VII Being Released in December 2015

For one I would to think STAR WARS movies are that special that they shouldn't just be put out every year quickly. That pace works for some franchises but for others, a few years feels more fitting.
If someone has that perspective, it is pointless to clarify. Remember, Lucas was pushing for digital shooting. I remember a quick TV interview from either late 1999 or 2000 where he said he would have liked to have four years between Episodes One and Two because he wanted the digital camera technology to improve. If you watch the featurettes you'll see how much time was spent on many sequences like the podrace or the opening of Revenge of the Sith. The techniques in the prequels were new at the time; it was a few years before the mentioned Transformers.
Regarding Transformers, if you really watch a Transformers movie, you'll see there really aren't as many computer generated visual effects characters and especially environments as there are in a STAR WARS movie. The release date of a movie isn't necessarily connected to the time it takes to shoot it. December 2015 was a very safe release to make from the time it was announced. It wouldn't have been anything between summer and late fall for marketing purposes so that extra six months has nothing really to do with the movie not having enough time. It just wasn't expected to be ready by May. The initial announcement of May was a little premature because nothing had been but the time taken to get the movie made isn't long. It just might seem so in an age where we can keep up with every detail for a few years.

 

For the PT, Lucas gave ILM one year to do pre-viz while he worked on each script. Then he spent four months shooting. Then eighteen months to do over two thousand effects shots. One year after filming ends, he spent two weeks re-shooting key scenes because of issues that he noticed after assembling the first cut of the film. Then the effects are finished, ADR work is done, editing is done, the score is recorded and all in time for the May release. Lucas gave his company that much time since he wanted ILM alone to work on the films. As opposed to "The Avengers", which had fourteen different companies working on the effects, including ILM and a shorter post production time, all in order to meet the deadline. Once again, ILM is working solo. Eighteen months is par for the course.

Now, if Lucas were doing the ST, he'd shoot all of what he needed the first year and come back a year later when Ford was ready and do the rest, plus re-shoots on other scenes.