PHPUnit jobs have been merged into one job. A few years ago we had separate categories that ran concurrently. They were merged for unrelated reasons, but it did slow things down.
Look into software that would help running these in parallel within the job (separate threads/processes)
- https://github.com/brianium/paratest (promoted by https://phpunit.de/plugins.html)
- https://github.com/verkkokauppacom/parallel-phpunit
See also:
T60771: Jenkins: MediaWiki extensions phpunit test should also run mediawiki core tests