In order to work around assetic in general with assetic:dump --watch like commands dig into Guard or Grunt ! I moved there because of similar performance problems and lacking configuration options in assetic.
The watch command has the downside of only being able to run with debug option.
Grunt/Guard aka. task watchers are able to automatically re-dump assets, run unit-tests and livereload your browser on changes to controllers or assets.
They even provide nice integration for GROWL (OSX / Windows) and libnotify ( *nix ) notifications for completed/failed tasks.
Use one of these in combination with CoffeeScript,SASS,LESS, ... source-maps for maximum frontend productivy.
If you use Chrome and Tincr with assets:install web --symlink you can even edit/debug your css and js directly in DevTools and save them back into your current bundle structure without leaving the browser.
Firefox has recently added source-map support in Aurora.
Assetic currently lacks the source-map options needed for various pre-processors but i'm working on that already ;-)