I have found an interesting tool from IBM for measuring web app performance from the client side: http://www.alphaworks.ibm.com/tech/pagedetailer
IBM Page Detailer is a graphical tool that enables Web site developers and editors to rapidly and accurately assess performance from the client's perspective. IBM Page Detailer provides details about the manner in which Web pages are delivered to Web browsers. These details include the timing, size, and identity of each item in a page. This information can help Web developers, designers, site operators, and IT specialists to isolate problems and improve performance and user satisfaction.
IBM Page Detailer places a probe in the Windows Socket Stack to capture data about timing, size, and data flow. It then presents the collected data in both graphical and tabular views, clearly showing how the page was delivered to the browser.
The page detailer is really good for quick assessments and one-off tests, when you don't have an existing suite of jmeter tests or the like to capture timing information.
This is a free download, although registration is required.