I'm having the same problem on my Nexus 7 2013. I tried setting location setting to high accuracy and battery saving. Disabled Wifi optimization and rebooted. I even wiped the cache and still it runs at 1.6Mbps.
Anyone find a way to root without buying an unlock code for assembled date 6/11/2014 and System Version 21.11.21?
Edit: Nevermind, I purchased a code. Waiting for it to arrive.