Ubuntu 16.04 on OSX with VirtualBox, Unity incredibly slow?

This is most likely caused by the absence of 3D acceleration (settings -> display -> enable 3d acceleration).

Unfortunately, at this time, this option does not work for mac. See this link

Have the same behaviour as ozma on my MacBook Pro 13" from 2017 with top specs. Enabling 3d acceleration improved the things a bit, but still far from perfect.

Start your Ubuntu VM,

go up-top to the main VirtualBox menu, click on Devices, and then ...

click on Insert Guest Additions CD images....

A terminal will open , follow instructions

... when complete, restart the VM.

NOTE

This answer is for macOS Mojave users who are facing this issue after they installed Ubuntu 16.04 on VirtualBox version 5.2.20 ... Nonetheless, others are welcome to give it a try :)