Videos running extremely slow

I received an iMac for Christmas and have recently set it up and what not. It's a second hand iMac, but it was hardly ever used so it's basically brand new.

I've been using it now for about 5hrs 30m straight, getting used to it and mucking about really. One of the things I've noticed recently though, is that my videos are really, really bad. They'll play for a couple of seconds, and then buffer for a long, long time so I end up pausing it and letting it load the whole video. It's even bad on 240p.

It was fine for maybe the first hour or so but started to get worse as the night went on. If it makes any difference, I don't yet have Yosemite, am currently installing TF2 and am installing a software update.

I doubt it's Adobe Flash since I use Google Chrome and Flash is already updated on there. It might be my internet (I use Sky lol) but I can watch videos fine on my Windows laptop and on my Xbox one.

This occurs for any type of video, however my Mac performance is still top notch (speed, loading, game peformance etc) but it's just that my videos suck ass.

Any suggestions, solutions, or recommendations would be very, very appreciated.

Thanks in advance

 

Sorry for wasting you're time, I think it just needed breaking in, it's fine now.