We're currently parsing all shapes eagerly as soon as we have loaded them. We could instead just send the unparsed bytes to the main thread and parse them once they're first needed for script execution or rendering. While this would move some processing to the main thread, it would save lots of memory in the general case and would improve startup speed because we wouldn't have to wait until all shapes have been parsed until we can show anything. @tobytailor, what do you think?

I made all parsing lazy in bug 1035170.