virtualbox.org

I have installed VirtualBox 4.3.20 on the new Retina iMac. I am using Ubuntu 14.04 as my guest OS.
Moving windows in the guest OS is very slow, i.e movement of windows is considerably lagging behind the pointer
movement up to a degree where it gets unusable.

did you install the guest additions in Ubuntu?

Yes I did install the guest additions.

k. attach the vbox.log file (compressed) for the guest.

btw, there is the possibility that Ubuntu will be slow even with 3D acceleration enabled on a Retina iMac. VirtualBox doesn't really support Retina displays nor does Ubuntu. If there is a way to lower the res to 2560x1440 that would help.

If I change the resolution for the iMac screen to largest text. It works as expected (almost).

almost is probably as good as it gets.