I am still using sonar-java-5.10.1 because any upgrade to sonar-java-6 ends up with 4 times longer scans.
Recently I tried SonarQube 8.9.2 LTS with sonar-java-6.15.1 with the same result.
When I replaced sonar-java-6.15.1 with sonar-java-5.10.1 in /lib/extensions directory it started to work good again. The difference is between 1,5h (old jar) and 6h (new jar).
We are using SonarQube in TeamCity, sonar scanner version : 22.214.171.1248
I see that there were some problems with performance in early sonar-java-6 versions but were supposedly fixed in 6.3.