When you are working with a PageSpeed server module, you sometimes need to debug a filter when the result isn't what you expected. With enabling the debug filter, lots of useful information will be added to the webpage source via html comments.
Decisions made by PageSpeed
You will be able to see why certain files are not inlined or combined, what decisions are made and why when applying image optimization, and lots more.
Other useful information include timing data as measured on the server, including the time the server took to send the first byte into PageSpeed, the time it took PageSpeed to parse and optimize, the time PageSpeed spent idle waiting for optimizations and fetches to finish up, and more.
Configuration and filter description
Enabling the debug filter will also make PageSpeed write the configuration that was applied for this request at the bottom of the webpage source, including a description of the filters.
You can turn on the debug filter from the querystring (when allowed) by adding
?PageSpeedFilters=+debug to the url.