Preview freezes every time I restart it and open a file Every time I open a pdf or image file using Preview when Preview was not already open, Preview will freeze up for a few seconds. If Preview was already open, i.e. I already had a pdf open and opened another one, then there is no issue with opening the second pdf. I've tried resetting Preview by deleting its files in ~/Library, but the problem still persists.
As a troubleshooting step I would boot your Mac into Safe Mode.

Boot into Safe Mode

Follow these steps to boot your Mac into Safe Mode:


Fully shut down your Mac
Restart your Mac
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
Take a note of what happens (i.e. use Preview a couple of times or so to test it)
Exit Safe Mode by restarting your Mac as normal
Log in and test Preview again


Let me know how you go.