The raspberry PI comes with default settings that auto adjusts the resolution. My problem is that my screen has FULL HD (1920×1080) that can slow down the Raspberry.
A 1280×720 is enough for me so I did:
> sudo vim /boot/config.txt
# uncomment to force a specific HDMI mode (this will force VGA)
You can check the full spec: http://elinux.org/RPiconfig#Videomodeoptions