In recent times I've been using the PSP Audioware VintageWarmer 2 on the master buss only, but I've also used it for mixing, and either way my system has been able to handle it. It doesn't seem like it will take up too much of your processing power to begin with, but of course it helps to have a stable system. I've got Pro Tools 9 running on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM, but I've also used it on other systems without problems. I'd be surprised if anyone had issues related to performance with this one...
Those in the market for an analog saturation tool to warm up your digital recording need to check out PSP Audioware's VintageWarmer 2. While it's not as good as the Crane Song Phoenix bundle, it's still very good and is great for both mixing and mastering. It will also cost you considerably less than Phoenix. In my opinion, no plug-in is like using real analog gear, but in the absence of having that type of gear, Vintage Warmer will take you a long way. Definitely check out the demo to get an idea of what I'm talking about.