there is a pretty significant issue that no one has touched on...you are doing all of these tests on a decent modern computer, decent video card, and a decent amount of ram, all by today's standards.
BUT...in 1998 the top of the line computers topped out at ~500mhz, the video cards to have were the TNT or the voodoo2 (tnt2 and voodoo3 were release in 99), and 256 was an ungodly amount of ram that only people with more money than sense had.
I would certainly hope that a computer 7 years newer can achieve the results that it does in GPL, and "struggle" through the newer games. they have more polygons, larger textures, perform more physics calculations, and do video effects and resolutions barely dreamed about in 1998.
GPL run on a 1998 equivalent of your computer wouldn't fair nearly as well as it does in your tests. hell, i'd wager (not a lot) that it would actually perform worse than the "modern" sims did on your computer.