The performance team, especially jwhiting@redhat.com , have been profiling a workload which is heavily using Narayana and Hibernate; while doing so we shared performance metrics for analysis and we figured that the following method is causing locking contention as it's heavily used and marked as `synchronized`:

com.arjuna.ats.jta.TransactionManager#transactionManager

Interestingly, it seems that this synchronization is actually unnecessary. I might be wrong, but I'll send a PR to test and discuss further.