Well, okay, but... why is it running better on the browser than via the launcher?
My guess would be because the browser version of Java is tailored to its environment. Speed on the internet is key, if your site and/or online software is slow, then people will likely get bored and use someone else's site/service/whatever. As a platform for stand-alone software, they could afford to take hits to performance...used to be, this was because they were really the only widely-accepted cross-platform, interpreted language around. Now, its simply because Oracle's slow to adapt to a changing environment, and if they continue like this, Java will soon fade into obscurity while its primary competitors, Python and .Net, take over where it left off.
[ETA]
Okay, okay, you motherfuckers have to see this.
I decided to pop into a randomly-generated MystCraft world in Technic, in my increasingly frustrated quest to find cacti (for waterproofing), as I spawned in a fucking taiga biome the size of Texas. I eventually found myself here, in the age I've appropriately called "Yawning Abyss." You're seeing that right, I'm at layer 1. I am literally mining one broken block away from falling into the void.
That was the most tense mining experience I've ever fucking had, but I got 18 diamonds out of the deal, so its not like I came away empty-handed.