I want to share what I figured out, and trust me it took me very long:
Ubuntu Unity likes a real graphics card, not virtual stuff from vmshare or virtualbox
It just doesn't work well in virtual box.
You can try all the stuff the other guy posted and it will be better, but not like you want it to be. Basically: don't waste your time.
Use another distribution that does not use Unity as UI.
I've successfully used Lubuntu 14.04
Not 16.04, because I did it a couple years back. And I think Lubuntu changed their UI, so be warned.
Another option is to switch to Gnome, but that switch has to be done completely, without exception... ( Ubuntu will go back to gnome in 2018 anyway )
I remember trying something like that back when I had my problems, and it did not work well neither...
There SEEMS to be an option to uninstall it via this.
A friend of me is doing that just that right now. I will let you guys know about it.
Use an image that comes with a different UI engine. Gnome, KDE, Lubuntu are some examples that should work. Lubuntu works for sure, at least in 14.04.
So my friend tried out and said Lubuntu 16.04 does feel a lot faster and more native than his Ubuntu install. But instead of reinstalling everything on Lubuntu, he tried out switching to gnome and followed this guide (mentioned earlier).
He said that after switching to gnome and removing Unity, it feels a lot more native.