MediaWikiIntegrationTestCase::addCoreDBData currently creates a user account and a page for every test (and for every Database test once r938440 will be merged). A lot of tests probably don't need this at all, thus resulting in a relatively large overhead.
We should stop doing that, and instead incourage the use of getTestSysop() / getTestUser() and getExistingTestPage(), which create the needed resources only if the test needs them.