Using youtube slows down my computer

I had this problem before when I had Ubuntu Desktop 14.04 64bit LTS with LVM and disk encryption.
I remember writing some command in the terminal which showed that it's not a RAM problem but a CPU problem.

I reinstalled Ubuntu and at the same time I installed 16.04 this time and WITHOUT LVM and disk encryption in case it was that which was slowing the computer down.

But the computer still slows down.

In software & Updates > Additonal drivers it says that it's using the x.org server Nouveau display driver (open source) which I've read is the best driver to use on linux if you having Nvidia GPU.

I have Intel Core i5-3230M 2.6GHz(3.2GHz) and Nvidia GeForce GT 630M with 1GB Dedicated VRAM.
I also have 8GB DDR3 Memory (RAM).
I have a 500GB HDD. No SSD.

Please, help me.. this is such a crippling problem :<