cassette rebuild is slow

I have a fairly standard web application (coffeescript, angular templates, a number of js libs) and when I run it in debug and make an edit to one of my files, coffeescript or angular template, it takes on the order of a minute for my page to reload. I turned on tracing, and it looks like every time any file changes, cassette rebuilds all of its bundles.

Is this intended/is there a way to limit the rebuild to the file in question?