There are so many possibilities. As Steve said, Internet speed is the most likely. It's the common link, as you have two machines with the same problem. Just go to a speed testing site and check. It's very quick and easy.
If you really have Vista on the PC, then it's quite old and also not the best operating system Microsoft has turned out.
You don't say how the machines are with other applications, speed-wise. (how quick does it open Skype, for instance)
A windows 8 machine would be relatively new, so unless you work it really hard, shouldn't have speed issues yet. All this, is of course also dependent on processor speed, amount of ram etc.
Also, your browser has already been mentioned. Although, Internet Explorer is used by many. It's worth trying others. Firefox, Chrome, Opera and many more.
If you are going to do a clean-up, give Glary Utilities a try. Always, when you install a free app. Do a custom install. Watch out for ticks in boxes. Just install the app, nothing more.
You also don't say if this is a new problem. There are many places in the UK, where speeds are less than 1 Mbit. Some are even still on dial-up. What I'm trying to say is, web pages are getting bigger and more complex, so demand faster access.
