Tyre wear and AI straightline speed

That's the whole point behind the tire wear. Not be able to push a whole stint. Its realistic and adds much more strategic possibilities. 

Stop running the lowest downforce setup, AI are not actually being physically faster than you on the straight, they are fast on corner exit, and with an unnecessarily low downforce setup meant you're sacrificing heavily of your traction, making matter worse.

Imagine yourself being 20kph slower on corner exit, you're going to had a roughly 20kph disadvantage throughout the entire phrase of acceleration until the AI reach their top speed because every car in the game technically have very similar level of acceleration. You are not going to recover from that disadvantage unless your car had way superior power output performance - which is unlikely.

And, because you're slower on corner exit, they will be gapping you throughout this race to the top speed, making you unable to take full advantage of slipstream for the entire duration, further worsening the problem. 

You're also very unlikely to ever reach higher top speed than AI with low-wing setup, why? 

First, because of the problems mentioned above, slower corner exit made you unable to close in to cars ahead to take slipstream, whilst everyone else behind you are taking yours, this basically already nullified every benefits that could come with low-wing setup.

Second, most straights in this game aren't long enough anyways, you'll barely find any advantage by only being briefly faster at the end of each straight. 

Not to mention, your low-wing setup will be making your car very slippery, all those excessive understeer and oversteer are gonna absolutely eat away your tires. 

Everything above is a snowball effect.

If you have 50% tyre wear after 3 laps it might be worth looking at the tyre pressures as this can affect temperature and tyre wear