Source.queue and SourceRef. How to decrease queue buffer?

I want to send data from my Akka service to another via Rsocket, which supports backpressure.
I have a handler where I receive actors SourceRef, merge them, and send asFlow to rsocket.
The problem is the consumer is too slow and I should batch messages.
In actors, I use:

val source = Source
                .queue<EventState>(1, OverflowStrategy.backpressure())
                .conflate { arg1, arg2 ->

And on each update message in actor I send some message in source:

source!!.first().offer(state).toCompletableFuture().thenAccept {
  "EventActor: ${persistenceId.entityId()}. Send eventState: $state to queue, result: ${it.isEnqueued}")

The problem is the source start conflating message only after receiving 53 messages, as I mentioned the consumer is very slow.
Can you help me how to create a source with buffer 1, which will merge messages if the buffer is full?

Hmm, did you try with bufferSize = 0?

From the Doc:
The buffer can be disabled by using bufferSize of 0 and then received message will wait for downstream demand unless there is another message waiting for downstream demand, in that case offer result will be completed according to the overflow strategy.

See also this discussion: