I had the same problem, ever since I installed High Sierra exactly a month ago. Slow screen shots saving to desktop. Slow ANYTHING saving to desktop. I did a test using this advice (to change where screenshots are saved to - I had them go to a folder in Documents) and my screenshots were then saving faster, it was a messy work around. But I need things saving to my desktop quickly, that is just the way I work. I tried fiddling around with iCloud drive prefs, namely turning off Desktop and Documents folder, but that didn't work. I didn't have Google drive installed so that wasn't it. I kept seeing references about having 3rd party things installed being a bad idea, such as Mac Cleaner. A few days ago I got rid of Mac Cleaner. Still no change. Yesterday I deleted Malwarebytes. Still no change. Finally I deleted Sophos. Restarted..... It's like I've got a brand new Mac! Speedy start up, apps start up so quickly, everything has become much faster. And, screenshots, or any file, saving to desktop instantaneous once more. I am so happy.
I can't categorically state it was Sophos, or whether it was a combination of the three things I deleted which were causing the problem. I did a restart after deleting each one, and it was only after deleting Sophos that I got the miraculous result.
I was blaming High Sierra for all the glitches I had been encountering, but now I am loving it and think it was the 3rd party stuff. I might even now take the plunge and upgrade to 10.13.3.
Now I just need to sort out the multiple 'ejecting of external HD' message when mac is asleep and I will be even happier.
Would love to know if this helps anyone else.