Virtual memory lets you use hard disk space to store information normally stored in RAM. But it takes longer to access information on a hard disk than it does to access information in memory. So, it can decrease performance if your system has to rely too heavily on virtual memory.
To increase performance, choose the right size for your paging file (the area of the hard drive that stores virtual memory). Administrator rights are required to change the paging file size. Illustrator performs best when the paging file's maximum size is about three times the amount of installed RAM.