Ipad Air - slow download speed

Hi.

I'm having problems with my iPad Air (A1474). Wi-Fi download speeds are disaster. There is some tests :

iPhone 5 :

iPad 2 (A1395)

My laptop (Windows)

Finally, iPad which is making problems :

- all devices in same distance from router (6-7 feet)

- internet provider : Comcast

- router : Arris TG862G/CT

- speed is "jumping" - it's about 15-20 Mbps for a second and less than 2 Mbps most of the times

- my internet speed is 50 Mbps (maximum)

Some other facts :

- tried different hosts

- tried factory reset router/ipad, rebooting ipad/router and other obvious things

- same results when iPad is really close to router (almost touching it)

- same results in my entire home (it's not really big but router is in the corner and I have good connection with other devices in second floor opposite corner and same poor connection with iPad Air)

It's not benchmark related post. Browsing internet with this device is just annoying and streaming (HBO Go, Hulu) is almost impossible.

Do you guys have any ideas what can causing this problem and how to solve it ?

Thanks,

Peter.