This is a hard question as iOS 5 is a unknown; we don't know anything about it. But, based on how Apple have treated iPhone's and iPod Touch in the past; here is what I think:
iOS 3 - Worked smoothly and efficiently.
10/10 for speed and 10/10 for efficiency (battery life)
iOS 4 It is smooth - Working smoothly; however launching the music app and movies app take slightly longer. When watching a movie, in comparison to iOS 3 on iPad it, almost, has the same battery life; however, when multitasking Safari with 3 pages open and 4 of the inbuilt apps, the iPad running 4.2, has 25 minutes less playback time. Also, I have noticed that when you press the home button, when playing a game, the game lags for a few seconds, then closes; this could be due to the app needing to be put in the "pause" mode, so when you launch it again it is right back where you started.
9/10 for speed, as switching between apps is fast and 10/10 for efficiency, when no apps are running in the background; and 8/10 for efficiency when apps are running in the background.
iOS 5 will probably have something that covers the locks screen - like widgets. I speculate that:
iOS 5 will have 7/10 speed and 7/10 efficiency It will be smooth
iOS 6 will have 5/10 speed and 5/10 efficiency It will be acceptable
iOS 7 will have 5/10 speed and 5/10 efficiency; because, in iOS 7, I think that they will disable features, just like they have done in iOS 4 for iPhone 3G and iPod Touch 2 It will be acceptable
iOS 8 - no support
I think that this iPad will "last" one extra iOS than iPhones and iPads have, as the processor is very powerful. NO iOS will lag the iPad to death; for example, multitasking, works OK on my iPod Touch 2g; however, wallpapers don't so they removed both - even thought multitasking was OK.
