Stefan Kaes, author of the forthcoming book, “Performance Rails”, scheduled to publish in early 2007, has this to say about Rails performance …
InfoQ – Tracking change and innovation in the enterprise software development community
… [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 …

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Computational Theology : Weblog
if we can get both invokedynamic and full hotswapping for dynamically typed languages working, implementations of languages like Python or Ruby could use the JVM object model directly, and really take advantage of the great performance of JVMs. That’s the holy grail of JSR292.

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