Zoom Tiles are created dynamically starting at zoom 7.
Why is this slow? Landsat 8 hosted on AWS using external overview which required more GET requests than proper COG. Also the geometry stored in STAC metadata is the bbox of each scene, resulting on non-optimized mosaic-json.
What are the black/white borders? Landsat 8 hosted on AWS doesn't use internal nodata and then it resulted in artefacts when creating the overviews.
Link: awspds-mosaic Github repo.