My husband and I are having the same issue. I also spoke with my son and he is running into this as well.
We all play on Freedom of course. It doesn't seem to have a pattern. I set my difficulty and zoned right into one mission, Went to help my son with another mission and went back to leveling with my husband and we were stuck in a load for 13 minutes. The next mish loaded fine now stuck again in loading for about 15 minutes. The progress bar is moving... SLOWLY.... Annoying very annoying. Any ideas yet?
After a 10 minute load, i restarted the game. Seems it might be happening again as I type this. Grrrrrrr.