I've been keeping attention to processes in my laptop and gala is constantly in the top 3 places, many times using more then 100% of the CPU. I think that is what was been slowing down my computer, because other then that, i just use Firefox.. I've been using linux from less then 1 month, its normal to be slow as windows after some months of use?
It's not normal for it to be so slow. There might be a corrupt package. Try reinstalling gala.
Open synaptic package manager by typing it in the applications menu search bar. Let it load. Once done, press CTRL + F and search for libmutter. It will display a list of packages. If you right click on any of them it will display the option for marking package for reinstallation,removal, etc. Mark the following packages for reinstallation:
Click Edit > Apply Marked Changes. Click Apply on summary window.
Reboot your computer.
Please check back to tell us if this helps.
I had a similar problem with Gala (Elementary OS 0.4.1 Loki virtualized in VirtualBox 5.2.14 on a Mac with OS 10.11.6). After killing the gala process and running the command "gala -r &" in the background, the CPU shows a normal behavior again.
It's just a workaround.