OSX Sierra - WindowServer process hangs, high CPU, whole OS freezes
Wow I accidentally debugged this myself.

It seems after all that my problem was actually a side effect caused by the real issue which was: macOS Sierra changed how it handled defaults write NSGlobalDomain KeyRepeat -int 0, it no longer supported a zero value, and I had it changed to zero in Yosemite or El Capitan to make the cursor advance faster. To reset it back to normal parameters I changed the keyboard typing delays in the System Preferences &gt; Keyboard back to fastest (both sliders were dragged to the left sidecheck using $ defaults read  NSGlobalDomain in terminal
, most likely cause I upgraded from El Capitan and something changed in Sierra, and the keyboard was queueing input events as fast as possible, so the WindowServer process hogged the CPU like crazy) and now my initial problem is gone.

Maybe I should change the title of the question so that others can find this solution..