I lived in Llanddwyn last year, last year was the first year to install 'Wifi' 'eduroam' in all University buildings, including accommodation. The Internet speeds are good in any hall and you can expect 10-35mbps through a WiFi connection, you can also use the wired Ethernet option (I suggest you buy a Ethernet cable) as Bangor has fibre optic, I ran a speedtest and I was receiving 50-100mbps in halls (depending on what time of day it was). Considering it was the first year for halls to have wireless capability, it was really good. It's just going to keep getting better and better each year.
In terms of a TV, I had one with a TV license and I used it on and off, I had good signal with the aerial I bought, in the newer halls (I know Llanddwyn does anyway), in the kitchen there is a TV port which provides superb quality. The common room also provides a TV, which has a variety of channels on.
Yes, there are points where your Internet may lag, but that's expected, nothing significant though. If you have anymore questions feel free to ask, I'm a Peer Guide for the University and apart of the Computer Science and Electronic Engineering department.