Most answers here are wishful thinking...
Honestly, it's complicated. It's not just one thing, but at least a dozen factors which combined, noticeably contribute to application launch and resume times. Some factors always have a role to play. Some factors depend on how the last app you were using is programmed. Some factors depend on how the app that you launched/resumed is programed. Some factors may or may not apply depending on how you use your phone:
Average seek/read times from NAND storage, RAM bandwidth, CLR initialization time (a big factor), the amount of work done by the apps going through state transitions to and from being suspended, the time it takes to tombstone an app when the RAM requirements for a newly launched app can't be satisfied, whether or not an app supports fast resume, whether the app utilizes the WP8.1 APIs or the 8.0 APIs that influence memory resource management, the size of the app you are launching, etc etc etc.
Most of this stuff won't make much sense to you unless you're a software engineer. Not that it can't be explained, but I don't know how to explain it to non-software-engineers without writing a book.
WP8.1 should have the affect that we see the 'resuming' message less frequently, AFTER all apps are updated to use the WP8.1 APIs, but when we do encounter the 'resuming' message, the device will behave pretty much the same as it does today.