virtual disk image - file or partition

What makes a difference in performance is removing layers of virtualization, by using a partition you remove one layer (although a tiny one) and thus a performance gain is expected. However if you are sharing the disk among other VM the performance gain could be null.

The performance gain here is so small that it highly depends on your hardware configuration (read disk controller and RAID layout). My common knowledge says this only makes sense if the device (disk) is not shared between more than one VM, and the VM has direct access to it. Another option is to use a disk controller in a pass-through setup (your hardware and guest OS must have support for it), this way you remove two layers of virtualization and thus the performance gain should be more apparent.