Hard tyres here too for the race, they weren't much slower then mediums and didn't lose pace as much towards the end.
I just had a look at the replay Christoph, and I don't think there is one place where you lose a chunk of time, to me it seems like a lot of little things that end up making a big difference laptime wise. What I could see is that you're not aggressive enough midcorner with the line, you give away precious space on the inside which makes the corner radius larger.
So try to get more towards the inside, making less way and also getting a higher average corner speed. The default setup is also pretty understeery (if you're using that), a few little tweaks with the gearbox and toe help the car a lot.