Three test devices:
Using Speedtest.net
(ping, download, upload speed)
I haven't accounted for all potential factors. (different devices, potential for others to use the network at the same time...) Runs were done sequentially.
My internet is supposed to be 15/15. Verizon FIOS.
Lumia 830: (Windows Phone 8.1 Update 1)
Test 1: 83 ms, 15.30 Mbps, 10.11 Mbps
Test 2: 91 ms, 14.45 Mbps, 9.40 Mbps
Test 3: 83 ms, 12.94 Mbps, 5.32 Mbps
Test 4: 81 ms, 7.20 Mbps, 1.97 Mbps
Lumia 920: (Windows 10 Mobile build whatever-is-current-as-of-6/17/2015)
Test 1: 81 ms, 6.10 Mbps, 2.65 Mbps
Test 2: 82 ms, 12.69 Mbps, 6.73 Mbps
Test 3: 98 ms, 9.52 Mbps, 8.19 Mbps
Test 4: 83 ms, 9.99 Mbps, 9.43 Mbps
Lumia 635: (Windows Phone 8.1 Update 1)
Test 1: 0 ms, 13.12 Mbps, 14.36 Mbps
Test 2: 81 ms, 15.32 Mbps, 8.25 Mbps
Test 3: 81 ms, 15.18 Mbps, 8.91 Mbps
Test 4: None performed.