How do i stop runescape lag in hd?

Ok so here's my problem, everytime i try to play runescape in the highest settings whenever i push the down arrow keys to get closer it drops frame rates and can lag, also in pking worlds its almost impossible for me to pk in hd due to terrible lag.. My computer had 3gbs ddr3 ram 2.2 ghs amd anthlon II p340 and the graphics card is ati mobility raedeon hd 4250 and i think those are well above the requirements for runescape hd graphics and yet my cpu still struggles in hd. What can i do to fix this and make it so i dont lag at all. Or could another problem be my browser, i use google chrome and some times (rarely) use swift kit irc.