performance is not the issue. Hardware assisted virtualization is easy and better to implement, usaully only 10k lines of code for a very good execution engine. However, software virtualization is a magnitude of more code. Software virtualization could be faster than hardware assisted virtualization.
You can use our VirtualBox based VMLite to run the same XP mode VHD file downloaded from Microsoft, on any Windows machines, regardless of VT-x or AMD-V, and it's faster than Microsoft XP Mode.
Free download is available here: http://www.vmlite.com
There is also a thread in the top sticky area, you can read for more info.