on
Instructs an Observable that is emitting items faster than its observer can consume them to buffer these items indefinitely until they can be emitted.
Backpressure:
The operator honors backpressure from downstream and consumes the source Observable in an unbounded manner (i.e., not applying backpressure to it).
Scheduler:
onBackpressureBuffer does not operate by default on a particular Scheduler.
Return
the source Observable modified to buffer items to the extent system resources allow