I kinda solved it.
I decided to do it my usual way one last (haha) time and start with a brand new installation of fedora. (I used this also to get rid of windows. I will focus on linux completely.)
But even after the new installation the boot time was the same. That brings me to think about hardware. In another thread of this forum here, I found someone who just unplugged several usb-devices.
And here we go: after I unplugged the offical Xbox Wireless Controller Connector the system starts as it should.
Startup finished in 7.915s (firmware) + 2.049s (loader) + 4.195s (kernel) + 3.338s (initrd) + 10.558s (userspace) = 28.058s
graphical.target reached after 10.538s in userspace.
I don’t know why exactly. Maybe I messed somethin up as I tried to get the wireless controller running. But I am glad it works now and that I learned a little bit. 
Thanks again to everyone how took time to help me!