I’m also experiencing the same issue, and as I progress through my career mode, it feels like it's getting worse as the cars develop.
The AI straight-line speed is completely bugged and way too fast. On some tracks, even with ERS on overtake mode and DRS activated, they can still pass me with ease — it’s impossible to defend.
It really makes the gameplay experience frustrating and awful.
I’d also like to point out that when checking the R&D levels, this happens with cars that are clearly less developed than mine, with significantly lower aero stats — there’s no way they should be reaching such high top speeds.
When you combine that with the AI having insane traction out of slow corners (you can easily lose 7 tenths on certain tracks), the fact that they don’t suffer from dirty air at all (while we lose grip instantly when following another car — which I actually enjoy as a realistic feature, but the AI are totally unaffected), and their seemingly unlimited ERS (I’ve never seen an AI car with blinking rear lights indicating ERS recharge), I think this really sums up most of the major issues players are reporting.
To EA : I believe this year’s game is overall very solid, but this AI behavior is something that urgently needs your attention. Fixing it should be a top priority to improve the overall player experience.