Motion blur in Maxwell is really fantastic, allowing you to render very long motion blur of several seconds or longer, with virtually unlimited motion blur steps. It really is efficient considering you are getting full GI, 3D motion blur. BUT, there is always a but – if you want to render visible emitters it will take a long time to clear these areas of the render, especially if the emitters are small.
So for your viewing pleasure, here is a video tutorial showing you in detail what the problem is, and a few ways to get around it and render a lot faster. Also as a bonus, a way you can get multi colored light trails over time, from just a single MXS file.