I have done some research about improving Rails Applications performance. I would feel the the following few points need to be considered when we optimize rails applications for performance:
1) Avoid the use of dynamic URL generation (link_to, url_for) since rails needs to look up the routes table and that may take time. Just hard code the controller name and the action.
2) Try to avoid the excess use of helpers since it adds overhead.
3) You may consider to use Rails Bench to do some testing for you rails application performance.
4) You may consider to use memcached to cache your model and library computation results.
5) try to optimize your database queries. If you use ActiveRecord find, be careful from computation intensive sql queries that returns to you a lot of data that you may not need. The method find may run many select statements for you.