Computers are very fast. GPUs are way faster than you think. Of course, it doesn't feel like that way since all the abstractions and layers kill performance by a thousand cuts.
Remember, "Aero" was a thing almost 20 years ago and that was a graphically intensive window renderer at that time. Nowadays we could have 50x better effects without a problem but UI designers are not willing to.
Oh for sure, I'm with you. Old graphics work has this mindset of avoiding transparency passes stuck in my head. Certainly a small part of the bigger problem.
Remember, "Aero" was a thing almost 20 years ago and that was a graphically intensive window renderer at that time. Nowadays we could have 50x better effects without a problem but UI designers are not willing to.