After upgrading from 1.24.x to 1.26.2+ (including the latest 1.27.1) we noticed that importing XML dumps took about 20 times longer than before.
Debugging shows, that the code added to "WikiImporter->finishImportPage" in https://phabricator.wikimedia.org/rMW341dfa2587220c8e9dff5866036b3092ceb682c4 (lines 368 to 387) slows down imports massively: Calling "maintenance/importDump.php" we get between 8 and 11 Revisions/s imported instead of ~250 Revisions/s.
We use "importDump.php" to synchronize our main wiki (read-write, restricted access) to a number of read-only, "front-line" wikis. The main wiki sports about 45.000 pages and synchronization should run hourly. Users are not amused as this process now takes more than 100 minutes (instead of 6 minutes before).
Calling "initSiteStats.php" once afterwards would take less than two seconds -- if we even cared about page counter bling-bling...