With recent release of a new dps meter overlay and its feature to track nemesis spawns, I have done some testing on the ascension altar and here is what I’ve found:
“75% faster nemesis spawns” seems to actually mean 75% less required kills to spawn a nemesis (i.e. 4x faster spawns) which is either a bug or bad wording.
It also only works for your first nemesis (per faction per session) which is just simply a bug

For example in this screenshot, the beast nemesis has already been spawned after ~120 kills, and now is back to the default 400-800 kills per spawn, while aetherials have not been spawned so they only require 100 kills (normally 400+)
Same for death’s vigil, it has not been spawned so it’s at 31 kills here instead of the usual 100-200.
While the testing has been done with a 3rd party overlay, it matches perfectly what is happening in game.
One more thing worth mentioning is that sometimes, a white mob may contribute 1 kill, other times that same mob kill will contribute 2 kills. (I know different mob rarities contribute a different amount of kills, but this is not what I am talking about)
I’m not sure what causes this behaviour, but it seems to be caused by the ascension altar (this seems more in line with the “75% faster nemesis spawns” wording).
This has been inconsistent from my testing, and either all mobs will contribute either 1 kill until the nemesis spawns, or all mobs will contribute 2 kills until the nemesis spawns (i.e. there is no situation where 1 mob contributes 1 kill, and then the next mob is worth 2 kills)
