Stefan Kaes, author of the forthcoming book, “Performance Rails”, scheduled to publish in early 2007, has this to say about Rails performance …

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… [areas where] applications [fall] short of achieving good performance.

• choosing a slow session container
• doing things on a per request basis, which could have been done once at startup
• repeating identical computations during request processing
• reading too often and too much from the database (especially in conjunction with associations)
• relying too much on inefficient helper methods

On top of that, there are still some problem areas within the Rails framework itself, where I’d like to see improved performance in the future. Some of these can be worked around at the application level, some can’t. My favorites on this list are:

• Route recognition and route generation
• ActiveRecord object construction
• SQL query construction