Make sure that you don't simply convert a physical machine to virtual machine. A best practice is to create a brand new virtual desktop machine from the ground up. This way, you don't end up with physical-based elements in your image that might cause performance issues.
Typically, a physical image will have been updated and patched numerous times and will have become bloated and might contain some old "rubbish". Virtualizing it means that we will end up with a bloated VM and virtualized rubbish.