Hello,
I’m happy to announce that the v6.3 of the Java Analyzer is fixing most of the performance degradation introduced with the v6.0. According to the tests we run, analysis are almost back to the performance of the v5.14. If you don’t see any improvements while upgrading to v6.3, we would like to exchange with you. So don’t hesitate to report it on a dedicated thread.
Here are some figures from our tests:
You can see from these graphs that the move to ECJ was not the root cause of the problem. We had to rethink how rules were written to make them performing efficiently.
The Java Analyzer v6.3 is available for download now for SonarQube users and will be deployed in the coming hours/days on SonarCloud.
Big kudos to @Godin for the huge work! 
Alex