Have you created an SPA before? If so, start with what you know best. Spending 2 months learning how to write BackboneJS is probably going to add up to more time than the lag all of the users of your first iteration get combined. You can then change the parts that aren't working for you, and optimize for your use case.

If you haven't, I'm a big fan of using a MongoDB, Express, Angular, and NodeJS (MEAN) stack. Angular has a huge number of libraries you can use for transitions, and you can always use CSS animations and jQuery to accomplish anything not in there.