As in nearly all performance metrics, your mileage will vary (greatly). At some point I'm sure that the cost of loading external files will outstrip the benefi

As in nearly all performance metrics, your mileage will vary (greatly). At some point I'm sure that the cost of loading external files will outstrip the benefit gained from not having it in memory. You need to test this in your application. It should be pretty simple -- just write everything in separate files. When you feel you have enough done to accurate represent a (performance) profile of your site, do load tests. Then concat all the files together, and do a load test on that. In short, it's time we start thinking about performance as a result of architecture crossed with implementation. This means you have to include your specific business logic and data models in your implementation. You can't evaluate scalability issues without doing some testing in your specific case, if you want the results to have any meaning.