Help with MBP

[11'MBP]

I have a late 2011 MBP. Its a great computer for me and has lasted me a while. In a bid to get it to last longer, i installed a 500gb Samsung SSD, and bumped up to 8 gbs of ram. it runs as smooth, if not smoother, than the new MBP's. The problem I am having is that since updating OS X(currently High Sierra), the fan has been running a lot and my computer gets very hot. This never happened before except when i used apps like Skype or Google Hangouts, but now when I'm watching a youtube video, the fan starts running and it gets extremely hot. I have read online that downgrading my OS would help, I'm just seeking a second opinion as if this is true and if so, which OS would be optimal for my laptop? Thanks

 

[velocityg4]

Check the Activity Monitor to see what is utilizing the CPU. Be sure to view all processes.

 

[11'MBP]

I did and it said activity monitor was using it the most at 7%, then random stuff like mds, window server and hidd would spike to 20%. i have no idea what these things are. i've attached a picture. thanks for your help