Windows 8 Start Screen very slow after Guest Additions

I installed Windows 8 Release Preview on a VirtualBox VM, and it worked correctly. Then I installed the Guest Additions to get the correct resolutions.

Now, the Start screen is very slow, takes a long time to scroll and doesn't respond well to clicks.

I believe it's not my host, as it's a good machine (i7 CPU, 6 GB of RAM) and this specifically starts to happen after installing the Guest Additions.

The task manager doesn't show anything wrong (i.e. no processes pegging the CPU). Any suggestions?

I had the same issue. Assigning more CPU cores (I went up to 4) solved the problem for me.

Turning on "use host I/O cache" in storage fixed it for me.