@hashar and I briefly discussed this on IRC a few days ago. The coverage job used to complete in under 5 hours, now it's timing out past 6 hours.

Based on the slow test report, my hunch is that the new (see https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/20d06b34bbc95bb6f886a03b964f0f3a2239ec36%5E%21/#F31) AutoloaderStructureTest::testAutoloadNoFileScope() is too slow. From the doc block:

	/**
	 * Confirm that all files in $wgAutoloadLocalClasses have no file-scope code
	 * apart from specific exemptions.
	 *
	 * This is slow (~15s). Running it arguably renders all the performance
	 * optimisations above obsolete.
	 *
	 * @dataProvider provideAutoloadNoFileScope
	 */

My hunch that it's much much slower when run under xdebug.