are you using the DSM system on the virtualbox for windows ?
Initially, that's what I did. I installed virtual box on whs 2011, tested the deployment and was a little disappointed with the performance (hdd read and write was around 5-10MBytes/sec). I had a spare ex495 lying around and decided to try it out.
I installed the synoboot image onto a USB thumbdrive, placed the thumb drive in the server, placed 4 new hdd. booted it up, once the synology assistant showed the device on the network, I removed the thumbdrive, installed the modified pat file and about 5 minutes later, once the pat file installed, I placed the thumb drive back into the machine and everything was up and running again.
It took about 3 hours for the shr to finish optimizing the datastore but once done, its been flying.