I mean a tier 5 primitive set has like, 40-50 armor total or something which is pretty damn good. Its been ages since I played vanilla so I don't exactly remember.. I went and checked the xml's Primitive armor has 8-12.3 armor per piece from t1-6 base. with a +-20% randomness thing. so 32 to 49.2 armor. Dunno if the +-20 can go above those base values but I think they can. Which if so, max for t1 is 9.6 armor, and max for t6 would be 14.76. For 38.4 to 59.04 armor. Its also stupidly easy to make and has pretty good armor so its worth upgrading imo. Of course, you'd wanna use a set instead for the bonuses, but other than specific armor parts, its best to just keep privitive on for a while. I think there is a pair of gloves, the preacher gloves that are +damage to undead those are a good upgrade no matter the tier as its more damage to almost all enemies in the game.
I feel the zombies need to have alot less health like less than half they do now, and just be more numerious. I've played mods with massive spawns and performance is fine, so that excuse doesn't really fly. Afterlife uses a mod called walkersim, which has roaming hordes, that roam the map (in the background) and spawn once the player gets close to them, they never spawn in front of you but it simulates the zombies roaming and moving around, and they usually spawn at the edge of the biome and roam inside and around it. If you live close to wasteland and are in the forest you can actually hve radiated hordes wander into forest. These hordes on default settings are often 20-30 zombies in size, and they are very common in all biomes. Preformance doesn't really get hit to bad by this. But the mod maker for afterlife actually optimized the game better than vanilla has it.