In the time that I have been using the Softube Tube-Tech CL1B plug-in, I've been able to get some pretty good performance out of it. I have been using the plug-in on a Pro Tools HD system that runs on an Apple G5 desk top and I've never had a problems running a number of these within a single session when I want to. This being said, since I haven't run it on a Pro Tools LE system (or something comparable), I can't really say how well it will run on a non HD system. How fast or slow a plug-in will run really depends on the system that you are running it on.
Overall, the Softube Tube-Tech CL1B is a great sounding plug-in that is incredibly easy to use. While it doesn't sound as good as its hardware counterpart, it still has a great tone and is an easier and cheaper way to get this sort of tone. The price is pretty reasonable and I always love having the hardware interface look inside of a plug-in as I find that it makes it easier to use and is overall more user friendly. If you are looking to add a great sounding compression plug-in at a reasonable price, regardless of if you are familiar with the hardware Tube Tech CL1B, you should definitely check out this plug-in version. I would also compare this one to the TC Electronic CL1B plug-in as both are comparable.