Sorry for coming back on this so late.
The getParticipantNames function has been added to FastRTPS to get going with exposing node names. It’s clearly a one-directional implementation, saying that every time you ask for available nodes, you basically ask FastRTPS for available participants. I am not really familiar about the underlying specifics within FastRTPS to give qualified feedback on why it takes that long. There is no API currently for the rosgraph, notifying about node changes, only about topics (AFAIK).
I believe that FastRTPS has support for callbacks on node changes, but I have to look into it. For the time being, I would invite you to open a github issue for that particular scenario of yours, maybe posting your code snippet and we can pick it up from there.