Probably a shot in the dark here, but I'm having a problem where I have installed Ubuntu Gnome 15.04 on my laptop (no dualboot) and installed Virtualbox. I imported a previous Windows 8.1 VM via .ova appliance and installed a fresh Ubuntu 15.04 VM. Installation /import went great but the VMs seemed slower than they should have been and they crashed the computer numerous times requiring a hard reset of the laptop.
Anyone having a similar issue with their install? I wonder if it is just because Virtualbox isn't updated to support 15.04 yet.
Edit: While on the subject of virtualization, has anyone tried out Gnome Boxes? Its a preloaded program in Fedora 21 and it looked interesting, but was wondering how well it works compared to Virtualbox first.