From a performance point of view: test and measure! Find the worst bottleneck, and reduce it. Rinse and repeat.

NewRelic is a great tool to help you do this.

Common performance problems to look out for in Rails are:

* Missing database indexes

* Long running code (eg PDF generation, file uploads to a third party) that should be put in a job queue

* Innocent looking ActiveRecord calls that use N+1 queries or fetch way too much from the DB and the result set is further reduced in Ruby