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?