Here are some things to try (all of these changes must be done when the VM is powered down):
•Give the VM more memory (RAM) to work with. I bumped mine up from ~1gb to 4gb.
Right click the VM icon, Settings -> System -> Motherboard tab, move the Base Memory slider bar to give the VM access to more of the host machine’s memory.
• Give the VM more CPU cores to work with. I went from 1 to 4.
Right click the VM icon, Settings -> System -> Processor tab, move the Processors slider bar to allow the VM access to more cores on the host machine’s CPU. You can also adjust how much of the host CPU the VM is allowed to use (i.e. what percentage) by moving the Execution Gap slider.
• Enable 3D graphics acceleration to the VM. Right click the VM, Settings -> Display -> check the Enable 3D Acceleration tick box.
Update: I switched to VMWare Player because it’s much faster (even with a single core I didn’t notice any lag). It’s free for non-commercial use: http://www.vmware.com/products/player/playerpro-evaluation.html.