If you've ever experienced, like I have, painfully slow performance in Virtual Box on Mac, and you're using a retina screen, I have a fix for ya.
Granted, it's not the prettiest fix in the world, but that's because Apple is a bunch of cock-eyed nitwits who remove features from the GUI, but still keep them around.
The problem that I (and many others) have had, is that in order to see a single thing in the virtualised environment, you need to scale it to 200%. Whether you do this within the Guest OS or in VBox image settings, is irrelevant - it's essentially the same thing that happens under the hood. The reason for this is of course the Retina display. Now, this "solution" would be fine, except VBox has a hard time calculating 4K resolution on a virtual graphics card, and the result is PAINFULLY SLOW PERFORMANCE. To the point where it's barely usable for even the most basic terminal tasks.
Fixing slow performance in Virtual Box running in Mac OS with a Retina Display
The fix for this issue then, is to make the VBox application run in Low Resolution mode. A few versions of Mac OS ago, you could do that by simple right-clicking the
VirtualBox.app in Finder, choosing
Get Info, and tick
Open in Low Resolution. This is the feature that Apple removed. Except, they didn't. It still exists, it's just not easily accessible anymore.
So, close VirtualBox if it's currently open, and fire up the terminal. Run the following command:
sudo nano /Applications/VirtualBox.app/Contents/Resources/VirtualBoxVM.app/Contents/Info.plist
Now find the line that says
<key>NSHighResolutionCapable</key> and change
It goes without saying that this fix will disappear when you upgrade to a new version, but it's easy enough to do I suppose.
That's 100% the reason I'm even writing this at all - it's so I have a fixed place to go get the info when I need it next time.