RTMP video provides a means for video playback with ultra-low latency. However, most players will maintain a (small) buffer, so you may still see latency of a few seconds.
If you're wanting to test the latency imposed by the delivery layer, you'll probably want to factor out the players latency. With ffplay you can do so by reducing the probesize and telling the player to use the external clock for sync.
These are not production values, and in the real-world you'll likely sacrifice playback reliability, so should only be used for testing/debugging.
ffplay -probesize 32 -sync ext rtmp://[server]/[application]/[stream]