CNET managed to get some more information from Microsoft regarding the upcoming January/February update to Windows Phone 7, described by Ballmer during his keynote. While we know about copy and paste but the other new feature quoted as a "significant improvement in performance when loading or switching between applications" was left a little vague.
Aaron Woodman, director of Microsoft's mobile communications business, went over some of the details with CNET and it turns out to be pretty interesting. In short, front-loading, graphic intensive apps will see the most, maybe even dramatic, improvement, while more text-based apps won't see as much. Basically, things like XBox games and apps with locally stored information will get a huge boost. However, apps like Flixster won't see much of an improvement, only because they pull down their data from the web and that's a different thing altogether.
When given an example like Bejewled, the comparison between an updated and non-updated device was given to CNET. Evidently, the updated device loaded Bejeweled a full 15-20 seconds faster than the non-updated version, which Woodman appropriately called "dramatic". What is nice of course is the fact that developers need not make any changes to their coding--this is all on Microsoft's memory architecture and how it allocates resources, so technically every app will see a gain, just some more than others.
Finally, you can also sense the nervousness of Microsoft regarding app-updates. While there are many "unknowns" about the whole process, the feeling from this and other interviews on the matter is Microsoft doesn't know either: they taking baby steps to make sure it all goes smoothly and they are leaving themselves wiggle room to adjust if needed.