Load Test Early!
The following is an article on load testing by Chris Merrill.
"Customers occasionally ask us “How early should we begin load testing?”
The answer is to test something, anything, as soon as the architecture is available. Performance problems have a wide variety of causes – from a single line of code to a load balancer setup; from a database schema to a server config file. Early in the development of the software you can catch simple coding problems and fundamental architectural limitations that are much easier to fix before a lot of code has been written.
Now, a word of caution: Testing against a scaled-down development or test system will not tell you how the final system will scale. So as a tester, you must be careful how you interpret these results. Interpreted incorrectly, they could raise false alarms that undermine your credibility in the future. But early testing can uncover defects that limit the capacity to unreasonably low levels. When investigated cautiously with the help of the developers and admins, a number of serious performance limitations can be caught while they are still cheap to fix.
Chris Merrill, Chief Engineer"
A good article on load testing from webperformance.