its a mix of game optimize and hardware.
here is a test for you: copy the world file on your machine with a console command and time it. you can time it by setting the dos prompt to have timestamps ($t). eg prompt $t$g and then do the copy, its crude but it will do. Does that take 30 seconds or more (if it does, your disk has a problem or its 20 years old or something). If it does not, its the game's poor serialization of the save data.