More than 2% of MediaWiki API wall-clock time is spent in the call to UIDGenerator::newUUIDv1() in ApiMain.php, added in a2b85209e2 ("Emit new style API action logs into Monolog"): May 8th: 2.40%, May 7th: 2.76%.
This can probably be ameliorated once PHP 7.3's hrtime() becomes available. In the mean time, it would be good to avoid having UIDGenerator::newUUIDv1() in the path of every API request.