Are you using 1.5.4?
You could try splitting up the load time into separate scenes. E.g., load to a main menu, load first area, additively load next area etc.
You should be able to use asset bundles with Playmaker, you just have to be careful with global variables. Global variables are a contained in a single resource file so all Globals should be present in that asset from the start.
However, I don't have a lot of personal experience using asset bundles, but maybe someone can chime in with their experiences...