When measuring website performance, comparing two different page loads is a common and useful operation. However, there isn't much available to do it; some tools, like Fiddler, let you export the timeline waterfall as an image, allowing a visual inspection of two page loads to be done, but this lacks the ability to easily compare things such as when a particular request was made in each page.
A good intermediate solution is to save HTTP Archive files of each page load; these are JSON files containing the request and response timing information, headers, cookies and (depending on tool) contents. HTTP Archive (HAR) files can be exported natively from Chrome and Fiddler, and via the NetExport extension for Firebug in Firefox. Internet Explorer can export an XML transcription of the official JSON HAR format.
But what about comparing the exported HAR data? There is an online HAR Viewer but nothing to compare two HAR files. This particular issue came to light for me a few months back, when I was working on page load performance at work, but it really hit home when, at the WebPerfDays on Friday, "HAR diff" appeared on a list of tools that web performance people wanted.
Putting the two together, I have put my HAR difference tool online for all to use. Just paste in two HAR files (or just one to view it alone) and hit "Compare!". The tool summarises the differences, both in which requests were made, their timing information, and how the browser utilised its connections. The precise level of details depends on whether a proxy (like Fiddler) or browser tool (like Firebug) was used to collect the information, but most of the information is generally available from anything that exports HAR files.