Use asset host for cookie-free domains for components, that make your components load faster.
Using css sprite can reduce a large number of http requests, so it makes the web page loaded much faster. It it painful to composite a lot of images manually, do it automatically.
If you want to do a large data query such as finding all the 10,000,000 users to send email to them, you should use batched finder to avoid eating too much memory.
In a system like forum, the title and body is displayed on show page, but only title is on index page. You should use select in query to speed up the query and save memory.
Memoization is an optimization technique used primarily to speed up computer programs by having function calls avoid repeating the calculation of results for previously-processed inputs. In rails, you can easily use memoize which is inherited from ActiveSupport::Memoizable.
Use select with has_many and belongs_to on Active Record Associations
rails migration provides a convenient way to alter database structure, you can easily add, change and drop column to a existing table, but when the data in existing table are huge, it will take a long time to alter existing table, you should try to merge/optimize the db alter sql statements.