You can always switch to an in-memory database if durability is not a concern for you. However it would be a lot better to determine the cause of the performance drop.You seem to have profiled the engine. Can you share your results and, if possible, how you produced them?
We are using hsqldb in memory mode and the performance is still bad versus version 3.* we are getting about 100 times worse performance with a flow of about 10 activities (all java methods)
Using JProbe to see which classes are using up all the time and it is in DbSession
The time is being spend in RepositorySessionImpl.findProcessDefinitionByKey under SubProcessActivity.execute
and under EndActivity.execute( ) withDbSession.deleteProcessInstance