I am not sure how it works when you host from your own game, tbh you should get a dedicated server if you want a stable and safe game with your friends the hosting yourself thing feels like a hack and all depends on your internet and all those things, its not expensive at all (i am paying around 10euro/month for mine) and you can bring in the save you and your friends are playing right now so you dont have to restart the map, this might solve the lag/fps issues you seems to be having, however we all are getting fps/lag issues in the big bunker its just how that thing works a ton of zeds that insta respawns in a small area.
About high end computers and i have seen this a lot, people invest in great parts and gfx cards but they stop at 16gb ram, why? ram is super cheap nowdays and you can never have enough ram, just throw some more sticks in there you have room why not use it you never know where the bottleneck is going to end up.