Updated iMacs to OSX 10.13.3.... like wading through Treacle !!.... Solution c/o Apple Care
A few days ago I updated a couple of iMacs from OSX 10.13.2 to 10.13.3
Afterwards they were so slow at some Apple related jobs, it's like wading through Treacle.
Empty Trash took 20+ seconds, similar for screen grabs to appear.
After a couple of days getting nowhere, I contacted Apple Support.
They rang back, so a free phone call, considering any contact takes ages, as they take you through everything.
We tried various things, to no avail, SMC & PRAM reset etc
Starting to think I may have to re-install OSX from scratch, from Time Machine, and potentially lose 4 days work.
A Supervisor was called in, they sorted the problem within a few minutes.
She said to remove (uninstall) any Virus protection, I had ClamXAV and Sophos installed.
She said Apple built in firewall was all I needed, and is regularly updated.
Now for the final piece of the jigsaw....
She said to instal Malwarebytes for mac, which I did.
A full scan, which only took a couple of minutes, showed some malware & popups hidden away.
Offending items deleted.
.... iMacs now back up to speed, with empty trash & Screen grabs appearing instantly.