Inefficient/unoptimized would be an accurate description. I think it's important to add, for bethsoft games specifically, that the save includes all changes to objects, even if the player themselves didn't interact with them(e.g. Physics interactions, explosions moving things, npcs bumping stuff around), and also includes all NPC changes. Master files(ESMs) get loaded, then the save loads the changes it has baked in to the databases. So, when you load up a save that has traveled the world and loaded a lot of things into save memory, the engine has to sit there and reconcile all the changes with the ESMs, which can add up quick if you're playing modded.