It sounds like yours is a bit of an older Mac (maybe 2012?). While Mavericks is a free upgrade, its core technologies seem to make things more difficult on older Macs trying to run Fusion, even Fusion 5. On the other hand, Fusion 6 is a paid upgrade...a bit of the hype about that is its optimization, but those hinge on the newer Macs. I was using a 2010 machine and had to tweak my virtual machine (running Windows 7) in several ways in order to get an acceptable response, once I had both Mavericks and Fusion 6 in operation. On my new (<2 weeks) MacBook Pro, however, Fusion 6 runs awesomely.
I wouldn't be in a hurry to upgrade to Mavericks if you haven't yet.
You can, however, run Fusion 6 on as old an OS as 10.7.5. See Installation | United States for system requirements.
