My car, with NO aids, will slow dramatically in corners, much more than other users (judged by replays on fixed setups, like the Jeff Gordon racing school at Daytona). Very annoying, how do I get rid of this issue?
Try the tuning section. If I remember right Mad Finn Tuners has a nascar tuning and If you submit a thread requesting the tune in that section you might get more help.
ASM is switched on permanently since an old update, that is what slows you down. You can compensate for that by increasing ABS to 10, to weaken the braking effect. But the essential skill for those Nascar challenges is accurate drafting, and very smooth cornering to minimise the ASM kicking in.
This isn't really about the NASCAR deal, that was just an example, doing seasonal cape ring is slows me wayyyyyy down in the long left hand hairpin. Pisses me off haha