When I first upgraded to the dev preview of 8.1, my phone was actually really unstable and unresponsive. It was quite awful actually. So I eventually did a hard reset (Settings -> About -> Reset your phone) after running a full backup. Things ran much smoother. I eventually found there are a couple apps that aren't quite ready for 8.1 that was causing problems, photostream was one particular bad one. Remember, 8.1 is in beta, and most of the apps you are running have not even been tested on 8.1, let alone confirmed to be stable on it. But things seem to be running much better now, and it is nice and stable as I would expect. It is slower for sure (I see the Resuming.... screen a lot) but definitely stable. Give both your phones a hard reset.
And all devices get slower over time. With each iOS update, my iPhone gets slow. It's at the point now where it's SO slow it's painful to use. Instagram takes 7 seconds to load now. The camera roll takes forever to fill in the thumbnail tiles when there are a lot of pictures. And even scrolling through anything (browser, app, whatever) is choppy, when it used to be smooth. I just did a hard reset and it is still slow as hell. Not looking forward to iOS8.
So while the grass always seems greener, it often isn't. Most of the time you're just trading one set of problems for another set. All you gotta do it figure which set of problems you can live with and be happiest.
Yeah, this is the thing. Once the new firmware is released in sure it will make a huge improvement. And the 920 is already an aging phone. A great one mind you, but with the hard reset only removing your files and resetting the OS, not actually reinstalling it, it seems pointless because ill just be reinstalling all the same stuff back onto it.
And by the way, all my software is up to date. I'm big into updating things