Tman Bacon, I've added a 5 minute recording (link) so people can see how fast the spawns are. It's not the best as I have protected the area a bit, but it should demo how fast things come back. I have not changed any settings from the default.
Edit to add an a link to a recording.
version 1.19.81 on Realms
Normal difficulty
Spawning in the Nether is a mess and makes the nether almost unexplorable. All types of mobs can spawn immediately after previous ones have been killed. For example, clearing mobs in an open area, turning away for a few second and turning back will often result in the mobs having respawned. Ghasts, blazes, Wither skeletons, all types. The state it is currently in makes any mob spawners irrelevant.
The expected behaviour is that when mobs are cleared, there will be a gap between the next mobs spawning.
@Tman Bacon. it is not just in the fortress. It just so happens that I made tot a fortress.. which took a long time. I'll do another recording outside the nether portal. It's just not right.
You can skip the first minute of magma cubes: https://youtu.be/lMGuyJaXxnU