I suggest you find the forums at the Virtualbox site and ask your question over there.
The users over there will be better prepared to discuss such topics.
There are no forums here at OTN for Virtualbox issues.
I can confirm that there is no issue with Virtualbox 4.2 and Oracle Linux 6.3. I use it all the time. Performance should be near native and some aspects like disk I/O may even be faster. Your problem could be insufficient hardware, like missing x86_64 or hardware virtualization support. However, like rukbat wrote, this is not the right forum to discuss Virtualbox and your computer hardware here. I suggest to start with the Virtualbox documentation to verify your hardware is sufficient.
Actually, the same behavior with centos 6.3 x86_64 was noticed.
However, I used the same virtual environment with oracle linux 6.2 x86_64, it work fine. I used also with other OS like centos 6.3 32bits, soalris 10, solaris 11 and it was fine too.
@dude: could you given me some information about your kernel version (for oracle linux 6.3) plz ?