So, strangely, for app testing purposes, I actually own one of each model of iPhone and iPad ever made by Apple. The performance, as you said, certainly does vary. However, from my experience, there's been only one really bad moment: the iPhone 3G on iOS 4. I'd put the next worst as the original iPad on iOS 4.3 (it's better on iOS 5, actually), but even that was nowhere near as bad and still completely usable.

However, this is actually an interesting point about Android. While you might not get updates, you can be reasonably certain that it isn't going to break: if an update is going to run horribly on a device and carriers/manufacturers only update a few devices, you're probably safe. In contrast, on iOS, Apple uses code signing to actually enforce usage of the latest release — you simply cannot downgrade.

If you go through the forum threads, you can see that I am not alone in being frustrated by iOS5 on the original iPad. Even scrolling on the home screen is laggy from time to time. I have already disabled Spotlight for all except apps and most notification BS.

I wonder how much of this is actually down to the OS vs the apps.

For example, I have a very cheap Android phone. It's not the best, but it does a good job. But lately I've noticed that some of the apps are running much slower now after some recent updates - Maps, Twitter, Angry Birds and esp Facebook.

All these need to work in harmony to get the best overall experience, but at the same time work well for the lowest common denominator (me in this case!)