Tom’s hardware have another fascinating article covering the real world performance degrade of using various domestic anti-virus solutions on a typical home computer. The real world slow down seems to be very negligible, but in a business there are often file servers, and that exposes clients to new problems.
The typical situation in which real-time scanning is a considerable problem, is when you are accessing a file server over the network, and that file server is scanning all the files you are opening with it’s own anti-virus rules as well as your own client on your desktop. This usually causes significant degrade in performance, in particular with images, generating thumbnails, accounts packages or when previewing folder contents with details. I would suggest excluding the folders from being scanned by your anti-virus on the desktop computer, and keep it running on the file server. Any savvy admin should always look to keep everything protected, which sometimes that means protecting the users from themselves. Ensure permissions on the file server are as tight as required, so that if you do disable real time scanning on a folder, the inherent permissions that user has will stop any infections spreading at a file level from the users computer to the communal file server.