Thank you for this answer.
I don't measure latency, I just hear a huge one (something like ½ second) when playing with a midi keyboard connected to fluidsynth. I tried to set blk_ms to a lower value but could not get an acceptable result.
I may try again later but for now I'm giving up with NetBSD. Too many things are not working and/or a pain to configure while it just works with FreeBSD.
In fact I'm experimenting these days with various OS to find the best one for my audio usecases. As of now the best results I could get were with FreeBSD. Linux is usable but still less performant, even using a RT kernel. OpenBSD is useless to me since it has no realtime capability. I still have to try DragonFly but that beast seems not to like my computers and refuses to boot on them...