Since your mentioning it's black screen before the desktop shows up. Install a tool called bootchart, and reboot your system once or twice.
sudo apt-get install bootchart
Then you ofc reboot your machine
The boot chart image will appear in /var/log/bootchart/*.png with the username and host + the date as the file name.
You can safety remove bootchart after sharing the generated .png file with us to review, and see if something there is the cause of the slow down for you.