Well, my client is modded (FTB) and I run it with a custom launcher (FTB launcher) but the same problem occurs to me in vanilla+vanilla launcher. I have 6GB of RAM on my laptop. And I allocated 3GB for minecraft. When I press f3, it shows me that it does have 3GB avalible, as I allocated, but it limits itself to using 250mb only, so it trys to rise over 250MB, but after 10 secs it remembers it 'needs' to go down back to 250MB. This results in 1-7 FPS on a laptop that once ran minecraft with 120 FPS. I tried changing Xmx parameter on FTB launcher (It has a place for java parameters) but still- it always goes back to 250MB after 10 secs. I really wish to play Minecraft as good as I can. (Btw, I lowered down the video settign to minimum, and I used optifine to lower it even further.
Slow minecraft due to low RAM usage