Hi all !
For information, we've run some benchmark of our product and we noticed some contention on CompositeDisposable add/delete (from FlowableFlatMapCompletableCompletable .FlatMapCompletableMainSubscriber). These methods are in synchronized block.
We switched to standard FlowableFlatMap which solved the contention (but i think it is adding a bit more pressure on GC because of the queue allocation. But i did not check that point).
This is just for informative purpose in case you're looking for some area to optimize ;-)