I spent numerous hours looking into what appear now to be separate gzipping issues / non-issues with my site.
I swear when I started looking, mod_deflate wasn’t gzipping any requests going through PHP. It still isn’t for HEAD requests (eg curl -I --compressed), but now is for GET requests (eg the browser and curl -i --compressed). I’m pretty sure it used to work for the former, which led me to confusion in all my testing. I did all kinds of playing with AddOutputFilterByType and FilterProvider, ‘php.ini’, and other configuration fruitlessly and, apparently, purposelessly.
It still isn’t gzipping error responses, eg 404’s and 500’s. I know it did before because I was using a script to check the size difference when making some changes to my error pages in October. I haven’t been able to figure out why it no longer is. I worked hard to make sure my 404 page was small, and now the transfer is actually larger than my gzipped home page.