Thanks guys for the info, we're experimenting with the mBoxUpdate model now
mboxCreate will load content inline. Typically content will appear to load like any other piece of content with the downside that it will slow down the load time of your page slightly. Adobe generally recommends no more than 7-8 mboxes per page to minimize performance impact.
mboxDefine with mboxUpdate on domready will improve initial render time, but will cause test content to flicker. This flicker may contaminate your results (e.g. you may have a winning experience but the flicker drags down performance. Alternately, it may draw attention to an otherwise ignorable element). If you go this route, you are better off using Adobe DTM to deploy mboxes. It will automatically hide contents within an mbox and show on domready. You can also code this manually through an offer.
Alternately, you could use Target Standard or Target Advanced with global mboxes to cut down the number of content mboxes.
Are you working on Test and Target Standard or Test and Target Advanced?