Same issues going on here with mine and my wife's iPhone 6.
After I activated both of our phones yesterday, I set both of them up as new phones. No restore from iCloud or iTunes backup.
At first, I didn't see any problems. Then, after about 5 minutes, wifi was beyond slow on both phones. I hopped off of wifi and onto LTE and the same slowness was present.
I rebooted my cable modem and router and that fixed the issue...for a couple minutes.
I then decided to reboot both phones, thinking it might be something with iOS. After doing that it fixed the issue again...for only a couple minutes.
No matter what I do, both of our iPhone 6's are beyond slow on wifi or LTE.
Using the Speedtest app, wifi download speeds are 0.30 Mbps. On LTE, just a tad over 1.0 Mbps.
Right after reboot though, wifi download speeds are 15.0+ Mbps. On LTE, close to 15.0 Mbps. As they should be.