iOS Multitasking Lag
The reason for this delay is linked to how iOS 'freezes' background applications; the period of inactivity experienced is relative to the time it takes to unfreeze the desired application. For the Settings app, the time necessary to become active is negligible, so it feels instantaneous, but for more complicated apps (like Mail) the thawing process takes longer and becomes noticeable in use. Until that point, touches do nothing because you are effectively looking at a screenshot of the application when you were last open — the app doesn't receive any touch events at this stage.
Using the iPad every day, I, too, think this is one of the most annoying aspects of relying on the device for work stuff. While Open In would certainly top the list for many people, personally I am more annoyed by that slight delay that occurs every time I switch back and forth between two apps with the four-finger multitasking gesture. The most common scenario is copying bits of text or URLs from a browser and swiping back to a text editor or Evernote to paste them; after swiping, I can't bring up the Copy & Paste menu instantly, because, like Benjamin says, the app is “suspended” and therefore unresponsive to taps. It is a very specific and minor annoyance, but one that, over time, becomes a detriment to the overall experience. As Graham noted, this doesn't happen on a Mac, and it shouldn't happen on an iPad either.
I have also noticed that the iPad mini tends to “flush” apps from memory far more frequently than my iPad 3 did. This happens with any kind of app, but it's annoying when Safari or Chrome have to reload every tab even if I only switched between 2 or 3 apps; I suppose it's related to the inferior nature of the iPad mini's hardware when compared to the latest iPad 3rd and 4th generation models.
I honestly don't care about seeing “multiple apps” on the iPad's screen, but I'd love for Apple to find a way to make iOS multitasking less aggressive without compromising battery life, making the process of moving between apps instantaneous as it is on OS X.