Things seem to be faster and responsive enough after I turned off transparency, and I'm happy with performance in iOS 8. I didn't expect transparency to make a difference, because I would think the GPU would do most of the work for that and not impact CPU or I/O, but maybe I'm wrong. Previously, I was experiencing pauses in app switching occasionally, but even that wasn't too bad.
I haven't disabled much else. I have Reduce Motion = Off (the default) because I never found it made a difference in iOS 7 (seemed to give you the illusion of being faster, but transitions seem to still take the same amount of time -- and I like the transition effects). I have all the background stuff running like background app refresh, automatic downloads, location services, etc.
SwiftKey still comes up very slowly (but is fast once launched), so I'm switching back to the stock keyboard as default for now (but I still have SwiftKey configured and can switch to it if I'm going to type something lengthy). I'm not sure if this a problem with 3rd party keyboards in general, or just SwiftKey.
The camera app had a lot of pauses and freezes in iOS 7, and iOS 8 is the same. I'm suspecting it could be my fairly massive Photo / Video library which takes up the bulk of my space (I have 64GB model with about 8GB free). Maybe that impacts the camera app. Or maybe having more free space might help performance in general (that seems to be generally true with solid state storage).