I can tell you, I'm in the same boat. I can't really do more than 4 -5 hours of actual coding on a typical day. I've done days with 7 - 12 hours of coding but those are outliers. Also by Thursday, I'm pretty burned out. I rarely get a lot done on Friday.
We all are, but we play this little game with our employers so that we don't get fired.
My take is that you can't shoehorn creative work into your desired time slots. Not if the result is supposed to have any quality.
I have found that sometimes I just need to stare away from the screen and forget what exactly I have written in order to obtain a fresh perspective and spot any problems.
No way to speed up this process, and what's worse is that if I try to log only the time during which I did "focused work", the result generally doesn't go over the mentioned 5h 15min daily.
Every person I ever asked about this is in the same situation.