Qemu vs. Virtualbox CPU usage

Something I noticed about Qemu (without KVM) vs. Virtualbox, on my 1.6 GHz dual core Pentium D laptop, without hardware virtualization support:

- Qemu uses about 40-50% of one core's resources when installing Windows 2000, with occasional spikes to 80-90%.

- Virtualbox, on the other hand, spends most of its time pegging one core to 99%.

In terms of guest OS performance Virtualbox is better, probably because of the better graphics support. But what's with the CPU use? I thought that KVM-less Qemu emulated all its hardware, while Virtualbox (even without hardware virtualization) ran everything on the host CPU. So why is Virtualbox being a bigger CPU hog?