Video problems

Playing in real time in after not possible all the time and most time AE is required to cache the frames (green bar) before playing any preview. so I highly recommend following some essential course for AE to get understand how this software work. try https://www.linkedin.com/learning/ where you can find a lot of helpfull cources 

It depends upon your PC environment and the codec. I would suggest that you try to play the video by another codec or update your graphic card driver.