With the release of Blender 2.83 it is finally possible to adjust the bluriness of the background in material preview shading mode. That means we can finally find out what has been hiding behind the curtain of bluriness we have been looking at whenever we opened up the shading workspace for the first time.
The high resolution versions of the HDRI environment maps that are shipping with Blender can be downloaded for free from HDRIhaven.com in resolutions up to 16k:
The spheres above can be clicked and link to the corresponding download page on HDRIhaven.