As to the original posters wondering why video playback is so erratic on Zorin that is because hardware acceleration for video on Linux is a mess.
You have multiple approaches that are not cross platform or cross vendor so some software supports this, that & the other, linux web browsers often have GPU video decode disabled by default.
The good news is that the Khronos Group behind OpenGL and Vulkan have developed a Vulkan Video Support specification and its now working in development branches of the open source Mesa GPU driver but only for one AMD GPU at this time. The downside is it will take some time for the above to bear fruit with software/drivers/distros all being linked up.
There isn't a whole lot the Zorin team can do about all of the above but partnering with a browser like Brave or Vivaldi for a ZorinOS edition were it has Intel's VA-API GPU video decode enabled out of the box would help. This though is a matter of money.