I tried the Aristo showcase (http://www.cappuccino-project.org/aristo/showcase/), and it took 20 seconds to load on high speed internet connection w/ Chrome browser. I can understand developers' reluctance to use such a framework. If you've marketing a web app developed with Cappuccino, you're going to lose a lot of conversions with that kind of wait.
In fact I often have to wait for Gmail to download too.
They're just not framing the conversation well with the default loading screen. If it said something like 'this will only take a while once', it wouldn't be so bad!
It loads in 5 seconds for me but the interface has many bugs and fails to implement many behaviors of native elements: scrolling with mouse wheel doesn't work, using the elements with keyboard is impossible because it seems many of them doesn't even have a focused state. When trying to focus on elements with the Tab key it's messing up the whole scrollview because after that elements aren't working even with the mouse. Pressing the Enter key redirects to another page. So it's a pretty bad showcase and the other demos too have the same issues. (using Firefox on Windows)
Is this really written in Cappuccino? I get some crazy graphical glitches when I click any of the buttons or dropdowns on that page. I hope this isn't the quality they were talking about, but I will admit that looks like it belongs in a Desktop app.