It may well be a memory problem, but I'm not at all sure that I see it any less on my 128 Gig iPhone 6 Plus than on my first generation iPad 2. It is definitely a problem in the fiber of iOS7 and iOS8. This simply never happened before that. The likelihood that this is not fixable without an OS re-design means not only that it won't be fixed, but also that it won't be acknowledged publicly as a problem by the company, and who can blame them?
It is funny, though, re your suggestion #1, the one app that seems to behave well is Mail. It seldom does a total refresh, and if you double click the home button into the app switcher from another app, instead of seeing an ancient screenshot thumbnail of Mail, you actually see an up-to-date screenshot including any mail you may have just received. Somehow, that app plays nicely, but how?
I may not have been the guy who said he went back to his laptop, but that is what I did quite awhile ago, though I had hoped iOS8 would change that - it did not. My iDevices are great for making appointments on my sync'd calendar, for checking minor emails (without extensive docs attached), sending texts, playing games, etc. This IS an improvement to my workflow, but without my laptop, I am dead in the water. I take it anywhere I go overnight.
I'm sure I could find extensive workarounds in the quest to untether, but it's not worth it.
I think iPad sales are probably down because they last and are well-made, without enormous incentive to upgrade, rather than because of workflow issues, though it would be nice if the market spoke for us. Also, I am open to the possibility that I am clinging to old-fashioned work habits, though I have yet to see anyone employ a better way to get done what has to be.
As to your suggestion #2, I have stopped keeping multiple apps open at all for the most part, and that helps a little, especially on the old iPad, but surprisingly little, perhaps because of the memory overhead of iOS8 itself in that old machine.
At the risk of rambling (oops, too late), maybe it comes down to waiting to see whether tolerating severe limitations is worth it to eventually arrive at the waypoint Apple has set for itself down the road, or whether some of their forecasting is misguided and other companies will pull ahead in functionality. Apple has been pretty good this way over time, eventually delivering power users many of the features they had clamored for for years, while other less forward-thinking companies have foundered as times have changed. I guess we'll see.
P.S. I think 6.5GB is minimal "disk" space. I'll bet your iBooks would refresh faster if you could free up 10 or 15GB - this worked for my iPad.
