[ubuntu] Issue: Ubuntu 16.04 runs sluggish

Hello everyone,
I just installed Ubuntu 16.04 on my computer, and I am experiencing serious performance problems.
My computer is Acer Aspire 5750G with i5, 4gb ram, 2gb nvidia 630M gpu and 120 gb SSD. Before installing Ubuntu, I had windows 8.1 installed and experienced no performance issues. And I have had Ubuntu 14 as my main os before, too, and it was really fast, faster than windows.

A little bit more detalied:

Performance on Ubuntu 14, like one year ago:
Everything ran really fast, I could multitask like crazy, there were no slight bit of slowness.

Performance on Windows 8.1, today (before I installed ubuntu):
The same. The computer runs cool when doing stuff like browsing inteet, but it gets slowly hot when playing a game, but still, you can play like two hours before you feel the computer is overheating. No performance issues.

Performance on Ubuntu 16.04, today:
Even though the fan always runs at full capacity, the computer gets quickly overheated, apps sometimes hang, the computer gets really slow when doing something like, opening the browser, coecting a hard drive. The transitions-app animations are laggy, sometimes the mouse animation looks like at 20-25 fps.

Do I need a fresh install of Ubuntu 16.04, or should I go back to Ubuntu 14 where it ran perfectly? Or perhaps there is a way without reinstalling Ubuntu.

If someone can help me, I'd really be glad. I am starting to feel helpless...

Note: One thing I have noticed is that the ubuntu .iso file that I have downloaded directly at ubuntu's website looks like this, "ubuntu-16.04-desktop-amd64.iso" although my cpu is intel. Maybe this has something to do with the issue.