Make sure you run jake deploy
to create the stripped and precompiled version of the app you're testing. Most sample code out there will be run in uncompiled "debug" mode which is great when you're developing but in actual deployment you'll want the precompiled version of your app.
Also, the current development version of Cappuccino and Objective-J is much faster than the last release 0.9.6, thanks to a new, better compiler. Keep an eye out for a future Cappuccino 0.9.7 release.
I think if you write a significant app in Cappuccino you'll find the load time to be absolutely comparable with other large web apps such as Gmail.
Currently you can only use Xcode to edit the user interface of a Cappuccino app. It does not work well to edit Cappuccino code itself since the latest versions of Xcode don't include the necessary plugin framework. You can learn more about good editor options in the development environment tutorial.
I am not familiar with other languages similar to Objective-C for the web, but Intel has an Objective-C to JavaScript compiler here.