Finder freezes when showing images quicklook preview
I would remove all your Quicklook plugins, then return them half at a time to eliminate the culprit.

They can be in 3 different places, /Library/Quicklook/ /System/Library/Quicklook/ &amp; ~/Library/Quicklook/

The fastest way to eliminate is by halves.
Remove half [remember where they came from]
Test.
If fixed, remove that [clean] half &amp; replace with half of the rest [the currently suspected 'dirty' half].
If not fixed, remove half of your half - to new folders.
This way you  always know which is your 'clean' &amp; 'dirty' half each time.
Test.
Repeat until you have found the culprit.

If the culprit is an Apple plugin, trash it &amp; re-apply the current combo updater for your OS - 10.10.5 at https://support.apple.com/kb/DL1832?