I've been banging my head trying to figure out what the issue is. As a dev, I've had the beta for a while. The GM was terrible on WiFi; so bad, I had to use LTE if I wanted any connection at all. When I installed the public release today (and set up as a new iPhone), I had the same problem.
I used OS X's WiFi diagnostics to select a better channel for my wireless network, and that fixed it. So, until I read this thread, I thought that one of my neighbors (I live in an apartment) got a new wireless router that was screwing with my channels.
I'll continue to test it out and see if the slow downs come back.