I’ve installed Windows Server 2012 Release Candidate on my VirtualBox today. Everything worked fine till I installed the Guest Additions, it suddenly became slow and unresponsive from time to time, especially when using Internet Explorer. Because it happened after installing the Guest Additions, I thought it had something to do with the 3D acceleration, so I disabled that and now it works fine.
So, just disable 3D acceleration on the video properties and you’re good to go!
Egads! I thought I was going nuts after first experience with Server 2012 beta on VirtualBox – hosted on a Dell T3500 w/24GB RAM (and about 1 block away from Oracle’s HQ)
. THANK YOU for this simple performance improvement! 
1 reboot and it runs normally! 4:45am. Ugh. Time for sleep.
No problem man! Seems to be a problem with NVIDIA’s Quadro…
it solved the problem.. thank you