If you cannot detect wide disparities in merit from the first few pages, you are not much of a reader.
There are two basic parameters I judge SF by:
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Literary quality (as I would judge non-genre fiction like Doerr’s)
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How interesting and original the premise and setting are, and how well they are fleshed out via world building.
When #1 is weak, it is apparent within a few sentences at most. #2 should come into focus within a few paragraphs (if it doesn’t, that is itself a weakness).
These are not random excerpts, but rather attempts by the author and publisher to put their best foot forward. If they seem good, the entire book could turn out to be less satisfying than their original promise seemed to suggest. But if they start out weak, abandon all hope: they are only likely to get worse. Not that I would stick around to find out! (Whereas the ones I highlighted with positivity, I have added all of them to my Wishlist.)