in dev mode, the asset pipeline induces extremely slow page loads in development (2-4 seconds per page locally)
This has been the single biggest complaint against Rails 3.1 if you google for it. For e.g. take a look at http://stackoverflow.com/questions/8084006/serve-assets-dire...
Similar questions have been addressed by either installing rails-dev-tweaks or by pre-compiling assets - which is then a problem if you're actually working on the UI/JS - which is where half the dev time is usually spent.