It makes sharing thread links viable. Otherwise my (paid) User Essentials add-on allows this sort of thing. You can then use my (free) Custom Field Permissions add-on if you want to gate it behind user upgrades.
I'm writing my own forum software (it's meant to be able to run with thousands of users on a cheapo vps- everything is as lightweight as I can pull it) and I ran into the same issue- if you want per-user posts/page settings, you can't grab shit by page, you gotta link by post, and then calculate the page. Extra calculations.
I'm still on the fence if it's gonna have per-user posts/page settings.
Where would you put it? Like, would you just slam it to maximum or what?
Because if it's gonna be a performance issue to have it be an optional selection users can change, then most users (probably the vast majority) are just going to leave it at default, and if the people that do want to change it just want to up the number of posts per page, then the simpliest and most robust solution might be to just change it so Threads always display by default the largest number of posts that are practical. Not as a user-selectable value, but as a global setting.