I think it's because the game stores each individual parasite the player has as one thing instead of having a count value for each parasite. I say this because every time I use the cheat menu to give myself lots of parasites the games loading time increases to about 30 seconds.
Twine keeps previous passages as basically a save state, every page you can rollback to takes up as much memory as the current one, and you get the entire list of saved pages shuffled around in memory with each click that moves you to a new passage. Then you have the necessary requirement for such a thing to work in saves: every variable that carries over to new passage has to be copied as a new variable. Only setup variables are spared from recopying, and temporary variables exist only in the current passage. Also, this breaks references to objects - in the current passage two variables reference the same object, but after getting copied over two variables reference two different objects, each with their own set of variables
So, if you as a coder want to decrease loading times, you have to decrease the maximum amount possible of rollbacks, rework how you switch between passages from "goto passagename" to using spans and replaces with displays on the whole page (but this will break saves because saves only capture the initial state of the page), unset variables, and so on so forth. As a player, you probably can use console to limit the rollback size. Can't remember how you call this stuff properly, sorry
