So I've had the same issue as everybody is reporting. I've always thought that 5ghz on the iPad Mini was a bit flimsy to begin with, but now with iOS 8 it's been awful. It's been driving me insane.
After I upgraded, slowdowns would happen so frequently it was silly. The only way seemed to be to "Forget" the wifi connection and rebuild it, which as you can imagine is not practical.
For some reason, these slowdowns have become less frequent in the last 24 hours. I'm also able to get past them by toggling airplane mode on and off. I think the reason it wasn't working for me before is that I was switching too fast. I find that once I enable airplane mode, I need to wait for the triangles at the top of Command Center to appear before I can disable airplane mode.
So far it's been workable, but annoying nonetheless and I've only been using my "workaround" for a day. I'm just wondering if anybody on this list had contacted Apple about it? Might be worth to report it if not.