Annoying lag when closing files

Hi everyone!

My Photoshop has suddenly inherited an annoying habit - there seems to be about a 5 second lag when CLOSING my images. I open an image, make a few brush strokes and close it down (even without saving, so it's not the saving process at fault) and the lag occurs. The image itself vanishes from the main window, but I can still see the image thumbnail in the Layers panel during which, Photoshop freezes. I can only resume working in Photoshop once that thumbnail disappears 5 seconds later. I've searched the forums and noticed that various users have had issues with this in the past too. I've tried all their solutions, but still it persists. What I've done:

*Optimised memory settings for Photoshop in preferences

*Tried changing GPU advanced settings within performance preferences

*Tried changing my virtual memory settings (although not sure if I allocated enough once changed)

*Updated my graphics card drivers

*Updated my version of Photoshop to the latest version

*Ensured my scratch disk is on a separate, empty and fast SSD within my pc

*Reset my Photoshop preferences

*Repaired all Microsoft Redistributable files

I'm not sure why this has occurred. It was working fine yesterday and today, it wasn't. I didn't add anything or alter Photoshop (or my pc for that matter) in any way so don't know why I'm being cursed with this. 

Has anyone else had this before and found a solution that I haven't mentioned? This is really putting the brakes on my usually quick workflow.