I have used the Soundtoys Tremolator plug-in on an Apple G5 computer that runs with a Pro Tools HD system and I can't say that I've ever had a problem running this plug-in. This isn't a plug-in that I have ever had the need to run more than two of at a time though, so I can't say how well it would run beyond this but I would imagine that most people wouldn't be running too many of them in a single session anyway. How well you can run this plug-in largely depends on the system that you are using, but if you are able to run the plug-ins you have now well enough, I don't think that this one will give you any trouble.
Overall, Soundtoys Tremolator is a very solid plug-in. It has a great overall sound and provides a lot parameters so you can really get all sorts of different tremolo type tones. The price is reasonable as well, so if you are looking for a nice tremolo plug-in to add to your suite, this is absolutely worthy of a trial. There aren't too many other tremolo plug-ins out there that I would rather use over Soundtoys Tremolator.