Youtube video lag on 18.04

I just upgrade from Ubuntu 17.10 to Ubuntu 18.04, there is an issue when playing youtube video.The video stops play but audio playing, then I move the cursor & video is playing...... What should I do??

Is the monitor refresh rate.    With me it happened in any video, that was in the browser or Totem, VLC etc.     Under Settings go to Devices > Monitors > Resolution. click to change. At this point there is the resolution chosen by the system and just below the same resolution. choose the second option and the Update Rate will appear. Choose the one from your monitor if not already ... 60Hz, 120Hz etc. Do the test.