As you read it in the announcement, the v6.0 of the Java analyzer introduced some performance degradation compared to the 5.14 and the 6.3.x was released to come back to almost the figures of the 5.14.
I believe it would be great to share more details about your scan time differences to confirm that the degradation you observed is really coming from the upgrade of the Java analyzer and not from another root cause (same build machine?, exact same code scanned?, …). Can you share privately the logs of both scans?
Also, I believe it would be good to try the latest release of SonarQube, the v8.7 were you will get the best performance we can deliver so far. I don’t ask you to upgrade your production installation, just try the v8.7 on a similar env as your current 7.9 LTS.