Actually we are both saying the same thing.
If you HAVE the original cup mod in the series folder you need the fast.cup.sim
If you DO NOT have the original cup mod in the series folder then you need a fast setup for the physics type you are using when you try to load the track, ie. if I have OutlawsGNS selected as the series and I do not have the original cup mod, then I DO need a fast.gns.sim in the track folder of the track I am trying to load.
Papy could have made things much easier by changing the requirement to base.<physics>.sim and when a user tried to load a track that did not have that default/base setup, it should write a base.<physics>.sim file to that track folder with a base setup so at least it exists... and not called fast so it would not be confused with an actual setup for that track, base would mean a bare neutral setup.