Basically put Mojang are a bunch of pretty shoddy programmers, they didn't optimise the code. But that itself isn't the big issue, its the language itself, java to be exact, frankly put java is very bad at handling resources; couple of thousand memory leaks here and a few bugs there. The only way you can get a good fps is to use a non integrated graphics controller, (not relying on the shared memory used by the i-series processors) such as a gtx 560ti or any equivalent gpu. There are a few mods which convert minecraft off java to C++ or C# which is much better with resources. There are a few ways to measure fps and change settings accordingly; here is what i do: Download fraps and enable the fps counter overlay, you may wish to reassign the key it uses to activate the overlay F11 by default; use this to change settings to your liking. Many people say you don't see any difference over 25fps (which is faster than the human eye can see) however you will still see drops in fps easily being so close to the border of playable. What is best is personal preference, me personally cannot stand below 60 fps (i play with v-sync off btw) as that gives me plenty of breathing room. If you use fraps it obviously measures fps but i can help with fps drops, i currently get 200+ fps and that is on a green background to signify that that is what i should expect from my gpu, however even i do fall suspect from drops, down to 30fps which is then on a red background, it quickly goes back up but that could be anything like drivers or temps.