The part in the last mission where you peek out of cover is really buggy. When you lean out, your aim starts pitching upward sharply and you fire uncontrollably (even when not pressing the button).
Cut down on the gratuitous particle effects. From the way the lag comes and goes I can tell you're at least clearing the data structures right, but the trash collector can't get rid of old ones frequently enough to prevent it.
Your suggestion of playing in Firefox to prevent lag did approximately jack for me. The lag seemed to be cascading over time, which means you probably have some data structures piling up -- are the notes, objects scrolling in the background, etc. being reused or nulled for garbage collection properly?
Some character types are unselectable, the translation is one of the worst I've seen in my life, the combat feels more like watching a fax machine than playing a game. The whole game seems exceedingly half-assed.
It has a lot of potential, but there are a ton of problems that add up -- poor balance and grind requirements among the foremost (snakes in the first level able to twoshot you unless you grind out three levels on beetles? Really?). Also, quest progress counts aren't reset when you die, and controls are kinda crap.
It's not just at default quality. It has some pretty bad lag issues even at low quality. Also, you should probably hardcode some kind of threshold of random attacks before the boss automatically uses the one that leaves him open -- I had to wait over eight minutes just sitting next to him in invisibility while he used the two moves that don't leave him open.
 "Cactus McCoy"){kind=icon}
 "de Blob 2 Revolution"){kind=icon}
 "Sierra 7"){kind=icon}
 "Guy of My Dreams"){kind=icon}
 "Bubble Tanks 3"){kind=icon}
 "Refraction"){kind=icon}
 "Flight"){kind=icon}
 "DJManiax"){kind=icon}
 "Arkandian Crusade"){kind=icon}
 "Santa Rockstar: Metal Xmas 3"){kind=icon}
 "Dragon's Call"){kind=icon}
 "The Visitor: Massacre at Camp Happy"){kind=icon}
 "Castaway"){kind=icon}
 "Toxers"){kind=icon}
 "Primary"){kind=icon}