Many web apps out there these days are using APIs or code that they've not worked with themselves, and quite often are using it inefficiently. Some google maps mashups come to mind.
With php/asp backends, HTML, javascript (especially with all the AJAX these days) - there are so many potential bottlenecks, problems, inefficiencies and double-ups.
What strategies can you suggest to investigating, identifying and resolving slow-downs, redundant code, leaks and other issues in web-apps - both server-side and in the browsesr on client-side?