Hero images has to be quite large with good quality.  Because of that those are not so quick to load. With lazy-loading Hero block loads first a small image and replace it with original one when it has loaded. 

Add that functionality with below steps:

  • open hero block
  • click "All propeties" on top right corner
  • tick option "Use responsive image (image size is as close as possible to the hero size)"
  • publish the block

 

See below example: