high cpu usage even without apps running

Dear Guys and Gals,

I am experiencing a strange problem, my computer was incredibly slow in the last days, after a little troubleshooting, I decided to simply format everything and proceed with a new, clean system. Sadly, this has not solved the problem, what is more strange is that my activity monitor register anhigh CPU usage even if I am running just safari with one panel open. With high CPU usage, I mean around 70%, even more weird is the fact that the activity monitor doesn't show any process using such a big amount of CPU power. To better explain, it's better if I simply attach a little screenshot.

Here are my specs:

MacBook Pro (15 inches, Half 2010)

2,4 GHz Intel Core i5

4 GB 1067 MHz DDR3

NVIDIA GeForce GT 330M 256 MB

I am running Yosemite 10.10.5

Thank you so much for your help! It is really important!