I did some googling and found similar complaints with older versions of Ubuntu/OSX, but none of the suggestions in those worked, seeing if anyone has any updated advice.
I am running VirtualBox 5.1.2 on OSX 10.11.6. I've installed Ubuntu 16.04. The VM has 8GB of RAM and a 40GB disk size. Right now I have 2D and 3D acceleration turned on for the VM, but turning them on and off doesn't seem to affect anything.
Symptoms are general lag involving anything with the Unity UI. Even starting with typing my password into the login screen. I type my password, then watch as the characters appear one at a time with about a half second delay between each one. Typing in universal search is a similar experience. Dragging windows around is also laggy. Mouse clicks themselves don't seem to be similarly delayed though. Typing in say, the address bar in Firefox is SLIGHTLY delayed, but typical of going through a VM and not nearly to the extent of the delay within Unity. Typing in Terminal is pretty responsive as well. Just seems to be actual Unity UI things.