Has been happening to me lately. Yesterday the loading bar progressed painfully slow. After about 10 minutes i loaded in with a framerate averaging 1.5 fps. Tried several sim restarts (3 I think it was) and same behavior. Turned off live traffic and it loaded straight in. The last time this happened a week or two ago the sim tried loading for over 15 minutes and I just got fed up and just killed the sim via task manager.
Did you manage to fix it?