No performance issues but you could go inside and not get your sleepers because max alive is met from too many zeds outdoors. Trust me everything adds up, and going back to the old way is not good for the game. We're trying to be responsible here. Outdoor zeds are still there, just not as thick. They respawn faster so all in all probably about the same, just not as many alive at once eating cpu.
Yeah, I certainly understand the technical aspect and why you've gone the route you have. I'm fine with it myself, I was trying to find some middle ground for those who want more z by simply making the new way a default setting that could be altered in a menu option, similar to how one can set the # of BM spawns now. Is there actually a compelling technical reason not to? If not then just do it, add the option, and move on.
Thing is I was playing an A17 mod and whatever the F it did to wild zombie spawning caught me by surprise, in a good way. I might only see like one or two at first, then more just kept showing up, far more than in vanilla. If I ignored them and got distracted in a city area they would amass to near wandering horde levels, it was nuts, and pretty neat.
Do you even check out any of the mods? I think I remember you saying you didn't, and I found that really surprising, especially given your history as a modder. I prefer and focus on vanilla, but check out mods too later on, it's super interesting. Just hope you don't fail as hard as minecraft has over the years, ffs I doubt they will ever make a full game, but with mods it can be pretty amazing. They got WAY too comfortable with that over time, it's embarrassing at this point, at least your dev cycles are far more significant with each alpha.