Air-Play not consistent!

I have the 3rd Gen of Apple TV up to date, and an ipad mini and iphone 4S running ios7. I live in Spain and therefore cannot access uK tv channels due to my IP, so I pay 3.99 a month to StreamVia for aVPN. This works excellent!. I can watch ITV Player, BBC Player, Film On etc.. perfectly on the Ipad. When I mirror the Ipad to the Apple TV it is terrible. It stops every 5 minutes and the wheel spins, sometimes it is better than others but in general its unwatchable! I thought it may be the VPN slowing down the connection but it would be bad on the Ipad then, and its not. Its only when i go to air-play and turn on mirroring? Ive rebooted the devices and even reset Apple TV. HELP!