clean install: check
Upgrade: check current with 8.1.3
Even reset settings after another week of constant reloading of web pages
My experience with IOS 8xxxxxx is that it is closer to unusable than perfectly usable. It works like a charm on same in house system on wife's Air 2. We gave her iPad 2 to one of our daughters and she gave it back saying she didnt have enough time to fool with it because it was too slooooooow and constantly reloaded web pages. Go figure!
Have you turned off Background app refresh and automatic app updates?
Also try turning transparency off - that helps a lot. Reduce motion can also help (But I leave it on as I like the animations).
I tested iOS 7 vs iOS 8 when iOS 8.0 came out, and iOS 8 was actually faster for a few things, but was overall a bit slower. Not massively slower, but definitely noticeable.