I was using over 4 times the amount of characters you are using, and by placing them spread out in a area / corner of a zone, i was able to create instant respawns usually anywhere, from vanilla to bfa. The mobs are "linked": just have to figure out which are "connected". Usually there are a few mobs that are required to set of the faster respawns, which are placed a bit away/hidden from the area you might be testing/farming. These spots you wont be able to find using less characters. Sometimes it can take a few minutes of killing mobs, they slowly respawn, then suddenly they start to instant respawn.
They do often adjust lootdrops and or nerf spots. Whenever i stopped getting multiple amounts of mounts every 15 min (instead maybe 1 per 30 min, if lucky), it was time to move over to investigate the next possible area "points 500 meters in that direction".
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