Used XP SP3 mode since Vista days.

Never use Vsync. (fps limited to 60 in vid driver settings; without limiting this system gives 150-300fps depending on track and number of cars; the only slowdowns are when navigating the menus in the default install)

Only time I've used windowed mode was ages ago when someone posted it might improve fps in flightsim; results were inconclusive on my system.

processes running in the background?!
This is Win11, it requires two dozen processes to open a text file.

But all three N2k3 installs run fine once on the track, it's dealing with this every time I start them -

With the two GTP installs I "OK" this and they run fine. With the default install I must immediately click on a menu option after "OK" or the game shuts down, when it is running all menu navigation is ridiculously slow.