virtualbox.org

Moved from 4.3.10 to 4.3.12 and I may as well give up on Linux guests!

Although windows guests work fine I have Linux 12.04 and a recent 14.04 Linux guest machines built that simply run at about 50hz

Completely unuseable!

Networking seems to be broken.

Can I back out the upgrade?

Or, why don't you find out what the problem is and fix it?

CPU can't just disappear, some task must be using it. Find out which one that is.