I have just upgraded to High Sierra from Sierra, and immediately I noticed that screenshots would take a good 10-15 seconds before appearing on the desktop. How can I fix this?
I have the same problem here. It seems to be a refresh problem. The file is actually there, it just doesn't show up. If I open a Finder window in the desktop folder the same problem happens. But I can verify it's a refresh problem because if I go to other folder and come back the file is already there. I have an iMac with a Fusion Drive. High Sierra doesn't convert it to the new APFS file system. I'm guessing it has something to do with that. My guess is that High Sierra is prepared for APFS and that APFS treats file refreshing differently from the earlier MacOS file system. So whatever signal it's expecting from the file system, it is not receiving.
I've had this trouble as well. From what I can tell, it's a problem with how iCloud desktop works in High Sierra. Screenshots saved quickly to iCloud desktops in Sierra, but in High Sierra there's a lag when saving to any folder controlled by iCloud. Changing the file type when saving a screen capture from Grab made no difference, but saving to a non-iCloud folder worked instantly.
I think somehow your screenshot location got changed to revert it back to the desktop typing
defaults write com.apple.screencapture location Desktop
There’s also discussion on this issue that could be related to finder slow to refresh or alternate locations for screenshots.
The common thread is a third party sync or backup program like Google photos, box sync, Dropbox, etc...