Pages VERY slow to load

[#1]

My inlaws are currently have real trouble with their internet. 
It is a fibre 80-20 product.  Pages are taking an eternity to load.  It doesn't matter if they are on a pc using Ethernet or a laptop on wifi.  A simple page such as the BBC can take 15 minutes to load which is clearly wrong, it didn't take that long on dialup LOL
Has anyone got any suggestions please?

[#2]

Ok so I called the support people and they suggested manually putting the Google DNS into the computers raher than using the Plus Net DNS in set in the router.
8.8.8.8 and 8.8.4.4
Did that and the machine now flies on the internet now.

Surely this is not right, is there some sort of issue with the plus net DNS or just their routers?
EDIT
Just googled and found their suggested DNS settings are as follows

Standard DNS server addresses
Primary 212.159.13.49
Secondary 212.159.13.50
Alternative DNS server addresses
Primary DNS 212.159.6.9
Secondary 212.159.6.10
Plusnet Safeguard - DNS server addresses
Primary 213.120.234.42
Secondary 213.120.234.42

Both my inlaws and my routers are both set as standard with the alternate DNS details.  Mine works fine and theirs is like a dog.  What the heck should people be using?

[#3]

That does sound odd. PN DNS servers aren't the fastest but they quite acceptable. I'm using them myself and browsing is fine. In general DNS servers have to be very slow to have a noticeable impact on browsing. Even more so since modern OSes will cache queries and most domains have a TTL of at least several seconds. It's worth noting though that computers usually default to passing DNS queries to their LAN gateway (your router) which in turn passes queries it can't resolve onto an external server. By entering Google DNS on each machine you're not just bypassing Plusnet DNS servers but also the DNS server in your router.
You could try switching their computers to use the PN servers and see what that performs like. If that is still fine it would suggest the problem is your router.

[#4]

I've had this issue for weeks now and even just now i tried to go on yourtube and it just hung like mad so i changed to google DNS and bam yourtube is working fine now.