Yesterday, I blogged about turning off the thumbnail on the Virtual Server admin interface for a couple of reasons, one being performance or load time of the admin interface. I found it in the admin guide, in the section relating to "Improving Virtual Server Performance". There's some other useful tips in there which I recommend you following. I've cut and pasted below, but also look at the sections about optimising hard disks. There lots of tips in the admin guide if you haven't read it thoroughly.
You can improve the performance of Virtual Server by making the following configuration changes on the host operating system: