Excessive CPU usage from client

The launcher is ok but when I start the game, I have a low framerate (5FPS) even in the menu. And if I launch the monitor system from Ubuntu, I can see that my CPU usage for Java is above 150% ! I noticed that the bigger the minecraft window is, the worst it is.

The command I use:
java -jar Minecraft.jar (launcher)

JVM arguments in my profile
-Xmx2048M -Xms1024M