I am running 10.9.2 Mavericks on a MacBook Pro with 8 G memory. When Fusion runs, after a while the free memory drops close to zero, the inactive memory goes way up, and the machine starts thrashing writing virtual memory to disk. Page outs goes up dramatically and performance is horrible. Spinning beachballs, unresponsive, etc.
If I use a tool like Memory Clean to increase the free memory, things are much better. But I should not have to do this.
The enclosed .jpg shows the state of memory.