I've been trying VirtualBox on several systems: guest is Vista, host is Windows 7, Ubuntu Jaunty, and OS X. The first two on the same machine, a 3 GB system with an AMD dual core processor, the second an iMac. Performance on Ubuntu is much slower than on Windows 7 on the same hardware. I copied an installed image. I then ran Windows update. It found 9 changes, primarily updating to Office SP 2. On both Windows and OS X hosts, the time to find and install the updates is reasonable. On Ubuntu I finally gave up after spending all day. Several times the guest was pretty much hung, i.e. display of Windows update would go blank. Configuration was as similar as possible, about 1.3 GB of memory, SATA I/O enabled, otherwise default.
Performance on Windows 7 is quite good. It's hard to compare with OS X because I have an older Intel iMac with maximum of 2 GB. That's not enough for both Vista and all the stuff I do on OS X. So performance was a bit slow, but I think it's consistent with the hardware.