Q: Mac Book Pro problems after installing macOS Sierra

After I installed macOS Sierra on my Mac Book Pro, I have had a lot of problems.  Here are some of them...

     1.  When restarting/booting.  I either get the circle with the line through it, or the folder with the question mark, or a solid grey screen, or if I am lucky the apple symbol with a line showing progress.  The last one leads to the login screen.  However, I have to boot it 5-20 times before I get the apple symbol.  The most common is the circle with the line through it that flashes the apple symbol every few seconds.

     2.  When I login (after successfully rebooting), the interface gets really slow.  I get spinning beach balls and unresponsive applications and frozen dock all the time.  Looking at the memory, I seem to have enough (8gb with only 3gb being used).

     I have enough empty disk space (about 300 GB), so I don't think that is the problem.  Any ideas?