Hear me out.
If your computer in ur opinion is good, then it absolutely can't be an issue relating to hardware!
Idk how good ur computer is. Might as well benchmark with a couple of intensive games and tell me ur results. :P
But I think I know ur problem.
I am confident that SL was meant to work well on chrome. I do infer that SL was made for chrome.
so the only suggestion I can give you is to give chrome a shot. Trust me, it's not bad at all.
It's a good browser. well coded with javascript. I must say that it is rather slightly behind the competition in graphics hardware acceleration, but on SL, it works mighty fine.
I know how much ur gonna hate it, but try uning chrome on SL.
I am confident that it will work.
Idk if this will help, but you might as well try clearing ur browser cache. Usually deleting ur browser history, download history, and file cache can help. you don't have to clear out the cookies. but i cant guarantee that this resolution will solve ur problem.
so the first shot you must take is to try out chrome. if it works well for SL, there u go.
Tell me ur results here so I can provide further assistance if needed.