Ubuntu 12.04 is much slower

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http://askubuntu.com – I have a 64 bit 12.04 installed in my laptop with 4 GB of RAM. But the response is not very good. When i run some application like playing video it doesn't play smooth or even typing right now is clumsy or even scrolling. However, the total usage of RAM is like 1GB and total CPU usage is about 50%. Is there any other factor that is slowing down the system? My system spec: CPU: core 2 Duo 2.13 GHZ RAM: 4GB Graphics: 512MB nVidia OS: Ubuntu 12.04 dual boot with Win 7. PS. I didn't have problem previously when it was Natty installed. (HowTos)