In ee753ba4 we started using <whitelist addUncoveredFilesFromWhitelist="true">. This is good for ignoring files we don't want to have coverage reports for, and including files that nothing touches yet. But unfortunately it also means that even if you run code coverage for only a single test, like
$ php phpunit.php includes/api/ApiQuerySiteinfoTest.php --coverage-html ../../docs/coverage/
it will go through every file instead of just the one that was touched. This is the difference on my machine between ~45 seconds and 9 seconds for the generation of the coverage files. It would be good if we could somehow change suite.xml to have addUncoveredFilesFromWhitelist="false" for when you're generating a few files, and true only for when you want to generate the full coverage report. If we assume that individuals don't often want to locally run the full coverage report, a simple solution would be to change it to false in the repo and have the bots that do the full coverage report patch it.