Personally, I do not think session management is going to help reducing MicroB startup time but if you are really into session management, I would suggest keeping a single DBus server in the system that takes URLs, loads, caches them, and returns local filenames ot the caller. Such a server will be really small and simple. Rendering will still occur inside MicroB app.

As to reducing startup times, a good idea would be to reduce library dependencies and config file complexity. For example, bookmarks are currently stored in an XML file. Storing them as plain text or maybe as gconf entries (whatever is faster) will save you on XML parsing and possibly remove the dependence on XML parser completely. Not being familiar with MicroB internals, I can't say for sure though.