I see no lag in my system, and Dalaran, WG, and AV load up in about 1-3 seconds on 5 clients (often 1 second). Although a RAM drive may be the future, an SSD is all you need for 5 boxing currently. I don't have symlinking, multiple installs, or anything like that, and I use the "cheap" RiData 32GB MLC drive.

If you have enough cores, RAM, and video card, then just get a small SSD and your lag problems will be non-existant.