Question: FINDER not showing previews and/or not responding (SOLUTION)
Finder was not showing previews of image files. Then it starting freezing (not responding).
This solved my problem:
By using Apple “Activity Monitor” App, I realized two applications were not responding: Finder and a third-party software (Adobe Creative Cloud). After solving the problem with Creative Cloud (by giving correct permissions to “/tmp” folder), Finder started working perfectly again. In your case, a different third-party software may be causing the problem (I heard GoogleDrive or DropBox were the culprits in other cases). So you may want to start tackling the problem by using “Activity Monitor” to check which applications are not responding at the same time.
Mac mini, OS X Yosemite (10.10.5)