Minecraft lags even though I have been allocating enough memory to minecraft.exe.

My minecraft lags even though i have allocated 4 gb of memory to minecraft.exe. further more my computer has 2500 gb of memory and 15 gb of ram. The game basically lags no matter what I do to it, except when I have the fog set to max and the graphics set to fancy. It would help the minecraft communtiy if you guys can help me.