So it's official that you are now lagging, what do you do you do? Well first off, do not panick! I have seen people with advanced and fast computers that sometimes do lag in runescape. When attempting to resolve the problem - one must keep in mind that there might be several factors contributing the problem, and perhaps not just one. Here are a number of possibilities;
First off, with the introduction of Runescape "hd" as jagex calls it, the game of runescape naturally became slower on most browsers becuase of the higher quality graphics. One tip is that you can first set your screen size to standard or normal instead of full screen. This might help quite a bit if you're playing in ful screen. Another tip is turn the graphic quality to minimum, so that your bowser needs to put less energy on the window you're playing on.
Another tip would be to play the game on another jagex aproved website like miniclip. If you didin't know, the jagex team has partenered with some affiliates to display their game on other game sites, and one of them is miniclip. Why this is important is because I have discovered that in fact, a lot of the advertisements on top of the game are responsible for runescape lag. Some ads being displayed are either too big in size, too flashy, and that causes major lagging. Another problem still comming from these banners is that when they rotate, they freeze the game for an instant or two (for some people more actually). And those rotations happen quite frequently actually - every few minutes there is one going on. So to tackle this whole ad problem, I play runescape on the miniclip website (I havent found any flashy ad or large in size, and neither rotating! on that website). To try it out, simply do a research on google about 'play runescape on miniclip', and it will come up with the apropriate link that directs you to play runescape on the miniclip site - you will be surpised how much this could reduce runescape lagging in general.
If the two above tips don't work, then chances are that the problem is comming from your own computer. You should go on a computer's friend for instance and see if he lags, or determine why. At this point, you would want to remove any mailicious software from your computer, delete all useless files and downloads that slow down your computer such as games, pictures, you name it. In some cases, you can even go and buy some extra memory sor speed for your computer if you have an old one, or just replace it. By that time, I suspect that you won't be runescape lagging any time soon - believe me, it will be history if you followed these steps!