When you are working with a PageSpeed server module, you sometimes need to debug why something isn't working out like you expected. Recently, Google's team has made a huge improvement on this, by emitting a lots of useful information via html comments when you enable the debug filter.
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.
One more useful thing is that enabling the debug filter will make PageSpeed echo back the configuration that was applied for this request. Another improvement here is that this will now also list a description of the filters, where previously this would only display short (cryptic :-)) codes for the enabled filters.
You can turn on the debug filter from the querystring (when allowed) by adding ?PageSpeedFilters=+debug
For an example, go to/?PageSpeedFilters=+debug and click view source.
The enhanced debug filter is available as of IISpeed 2.0 (PageSpeed 1.9.x).