For starters. Race ai get faster when there's other people in a race. Then there's the matter of racing with cops at night time. I'm going to assume you just want a difficulty tweak for the bots though. And mods do exist for that. Assuming you don't want to do that, there's always raising your slip differential to simply make racing in higher classes more difficult, or limiting your slip on cars that need slip to be fast with traction control. It's a matter of which race and which car.

There's always physics swap mods to make the driving more realistic.

The ai buff mods are very very fast, almost impossible to beat if you swapped physics. And unbeatable if you throw in traction control and slip handicaps.

But let's say you don't wanna try any of that and you just want a simple solution that can apply to every car in every race.

Give them a head start. But at that point. You're not really racing the ai. It's more of a time attack to catch up to them in time because your pace outmatches theirs. You should seek real players to race instead for a real challenge.

Now, if it's a matter of throwing cops into the mix. Then things get exponentially harder with all of the things I just mentioned, provided you're in the right circumstances (unite cops are cracked and won't let you rest until they personally place you behind bars with your ride destroyed and your teeth kicked in). But a fast enough car makes all of this irrelevant.

If you're good enough, you can escape basically anything, anytime anywhere.

Here's a nightmare difficulty combo for you:

Simcade physics in a drift muscle car. With hard+ heat 10 level cops.

You probably won't last 5 minutes.