I am currently running the McDSP DE555 on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I run the plug-in in Pro Tools LE 7.4 that runs off of a Digi 002R audio interface. With the configuration I've never had any issues at all running this plug-in, and I am able to run a few of them at once if I so please to, but I usually don't use this all that often and when I do I will only use one or two at a time. However, it is good to know that you could run a bunch at once if you wanted to.
I've been using the McDSP DE555 for about a year and I've found it to be one of the better sounding de-esser plug-ins that I have used. It has a good amount of parameters which makes it a very versatile plug-in as far as de-essers go. It is very easy to follow and use, while the graphs are convenient and come in handy. The price of the McDSP DE555 is relatively cheap as far as plug-ins go and it would probably be the first de-esser I would recommend for home studio owners looking for a cheap option that still sounds great. Overall, the McDSP DE555 is a great de-esser that has the capabilities far beyond that of de-esser plug-ins.