Even with the caching changes, a 150kB thumbnail still takes 1-2s to fetch. This impacts the score.
## Change
(1) Start with a large-enough placeholder image (has to be larger than the final image -- inflating doesn't count), then delay just enough for scoring, then switch to the real thumbnail.
(2) Since we are now doing post-mount stuff, we have the exact dimensions to optimize the claim thumbnail. This reduces the typically-several-MBs thumbnail to kBs.
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