Don't generate blurhashes at render time

Because the blurhash is calculated inside a hook_preprocess_image() function, unnecessary and costly regeneration of the hash happens after a Drupal cache rebuild or anytime the hook is executed such as during theme development when Twig caching is disabled. This severely hinders the theming process so as to be completely unusable on any page with an image of significant size.

Therefore, I would reiterate the importance of caching the generated blurhash either in the database (as issue summary suggests) or at the very least in the filesystem.