I was having the problem described in this question: Capturing screenshots on macOS Sierra is very slow. However, while searching I found Screenshot taking a very long time to appear on desktop first (particularly because I had skipped Sierra and upgraded to High Sierra) whose answers did not help me.
By further research, it seems clear that these are in fact the same problem — it is a Finder problem that can be triggered by the use of any sync software. Accordingly, I flagged the latter question suggesting that it be merged with the former.
However, I don't seem to have communicated the situation adequately using flags, so the new situation is worse — the latter question has been marked as duplicate, but not merged, so the existing answer mentioning iCloud as a possible cause will now be less prominent.
Therefore I am making this louder, public request: please merge Capturing screenshots on macOS Sierra is very slow and Screenshot taking a very long time to appear on desktop (or explain why this is not a good outcome).
(I can see it being arguably better to write a single answer covering both cases, since the ideal Stack Exchange question has comprehensive rather than partial answers, but I still think that merging is superior to not merging in this case.)
More history, not relevant to what I am requesting: The first flag I raised:
This is not a perfect duplicate of apple.stackexchange.com/questions/264463/… but I believe that merging these questions would be beneficial to being able to discover the multiple possible causes of this problem (e.g. mine was only mentioned in the answers to the other question, but this one showed up because I searched for "high sierra") – Kevin Reid 2 days ago helpful - Thanks. I’ve made sure to link the two questions. You can also help editing your post or answer to link here and explain the relevance.
I saw no edit or other action constituting "link[ing] the two questions", and I have no "post or answer" to edit on either question.
I then flagged it as duplicate, which was accepted, and then I flagged
This question should be merged with its duplicate, because the answers cover multiple causes of the same problem that might be applicable to different (this one mentions iCloud, and that one mentions Google Backup & Sync) – Kevin Reid 2 days ago helpful
but no action was taken.
I don't think that this is a situation that is extremely important, but I am particularly disturbed by the fact that the action I proposed was neither implemented nor rejected.