I was watching skilling videos and my browser slowed to a crawl...
Try not watching those videos at such a high quality level while doing many other things at the same time, or even just while having RuneScape loaded. If you're trying to push 720p or more while doing so, then no wonder you're having issues. Like, try 480p... and this IS coming from someone who's only on a single-core ancient system and knows what the true limits of 2.4 GHz are.
Also, use 'AdBlock Plus' ('ABP' for short) with your browser, and make sure to add filters yourself against advert domains and tracking scripts. Or you can use the alternative with ABP and simply subscribe to community-built filter lists that can take care of most of that for you. Either way you choose with that, you'll find very quickly that you gain a chunk of system performance back by not having the background junk of those advert domains/tracking scripts running. They're the biggest source of your problems when video-watching on the internet
~Mr. D. V. "Don't try to floor it so hard!" Devnull