I understand perfectly well what you mean. I put my system together myself in 2008, and overspecced it for my needs, so I have had the good fortune never to run into performance issues with any of the software I use. There is a whole underlying support system required to make use out of any of these programs, and that has to be considered in conjunction with the investment cost of the software itself.
That said, in my testing I did install Amplitube 3 on my wife's laptop, an AMD-based dual-core with a fairly low clockrate that you can't budge if you want to. Using both a StealthPedal interface and a Rig Kontrol 3 interface I was able to run at least two instances of Amplitube 3 in high quality mode without issues, at an acceptably low latency (comparable to standing ten or fifteen feet from a real amp in a room, my standard for acceptability).
It is a very modern and sophisticated piece of software, and as such if someone happens to be using an older computer, they could be in for a less than happy surprise if they read a review of the software and think their tone quest days are over for a relatively small investment. Perhaps I should have mentioned something to that effect in the review beyond noting its three performance to quality ratio settings. However, I do know from experience talking to folks on forums, on my blog, and in person that most guitarists who know and care about software modeling are pretty well versed in all of the factors involved in getting a satisfactory experience. Software just seems made to plug in to a studio environment (home studio, project studio, professional studio all included), where all of the underlying hardware concerns are already settled and the question gets simplified to "which program will offer the best features and sound quality for the best price?"
I do very much appreciate the feedback, and will consider what you said. Please don't hesitate in the future, it is valuable to me as a reviewer!