Can you be more specific? Are you talking about rails startup time or SASS compilation? Or something else?
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.
Anyway, yes I think that this release fixes those issues. However, there appears to be a problem with the SASS @import command and partials[1] so I haven't been able to fully test my system yet with all it's assets working properly.
curl get.pow.cx | VERSION=0.4.0-pre sh
If you are interested, I posted in the comments to this gist (https://gist.github.com/1184843#gistcomment-65875) some changes that we made to our SASS files to markedly increase the compile time.
For us it went from (depending on the machine) something like 40s down to a <5 seconds which was a massive improvement (CSS development was almost impossible for a while there).
They have indeed been crippling for front-end development, and majorly under-reported since this summer. So if you're facing any of these issues, please participate in the GitHub Issues threads I linked above and help the relevant teams diagnose/improve the problems.