I recently got a Macbook Pro (15" mid-2012) and have installed VMWare Fusion and set up a Windows 7 VM in Unity mode for the windows applications that I need for work. I upgraded the memory to 16GB and have dedicated 4 CPU cores and 8GB of RAM to the VM (I'll be running Visual Studio, SQL Server and some other heavy stuff there.) Running Mac OSX Mountain Lion 10.8.4.
I have experienced this slow mouse movement issue, and I can see one factor about my setup that could be contributing, and that's the DisplayLink adapter I'm using to connect to my external displays. I have a Kensington K33972US (sd3500v) DisplayLink adapter, and I have lots of issues like the ones described in this thread. I notice that some folks say they are running 3 & 4 external monitors, so I imagine they are using DisplayLink Adapters as well.
Today I updated to the latest DisplayLink driver v2.0 (DisplayLink: Mac OS X Software) and things are markedly better but still far from perfect. It's possible that some of these issues are due to bugs in the display link drivers or in OS X, based on the comments here http://www.displaylink.com/support/ticket.php?id=199
The one application that reproduces this for me most readily is WinSCP. It's greatly improved with the 2.0 version of the DisplayLink drivers, but it's still very choppy and blocky to move the window around the screen in Unity mode on my external display -- you can see the gray rectangle that represents the window as you try to drag it around. Moving the window to the built-in display of the Mac completely resolves the issue.
I'm pretty displeased with the DisplayLink adapter and may just return it. This might be helpful for other folks seeing similar issues.
--Joe