Quicklook and movies

[Poncho]

I've just upgraded my iMac from running Sierra to Mojave 10.14.6. All fine except for one annoying issue. When I use Quicklook to view an mp4 or .mov file on my external hard drive it won't load, or takes an age to load. Eventually a thumbnail will be generated and you can view the file as usual using Quicklook. But move to another movie and go back and the Quicklook function will be sluggish again. Anyone else had this problem and know of a fix? Is it because Quicklook has to generate thumbnails of all 400 movies on my external hard drive before functioning quickly? Do I have to open and play all the movies to help with this happening?

Have tried all the following suggested fixes to no avai (I don't use Dropbox in any case)l:

To get QuickLook working when previewing files with spacebar:​

1. Delete DropboxQL.qlgenerator in ~/Library/QuickLook, as per suntrop's suggestion.
   • Open Finder
   • Press ⌘+G
   • ~/Library/QuickLook
   • Look for a file named DropboxQL.qlgenerator, it will probably be the only one
   • Delete it, and then delete it from Trash
2. Restart Finder:
   • Press ⌘ ⌥ ␛ (Command + Option + Escape)
   • Find Finder
   • Click Relaunch

To get thumbnails to work:​

1. Run the Terminal command Dishoperator suggested: sudo update_dyld_shared_cache -force
   • It doesn't do any harm, it just updates the kernel caches
   • Open Terminal
   • Paste sudo update_dyld_shared_cache -force
   • Type in your password (it will not appear as you're typing)
   • Press Return
   • It will take ~30s to a couple minutes depending on your machine
2. Open up Activity Monitor (⌘+Spacebar, type in Activity Monitor)
3. Order by Process Name
4. Force quit quicklookd
   • Click on quicklookd
   • Press the X stop-like button in the top left
   • Press Force quit
5. Force quit all instances of QuickLookSatellite
6. Keep force-quitting until none of them are left
   • Make sure you order processes by name to see them all at once
7. Restart Finder (see point 2. Restart Finder: again if you forgot how)

 

[Poncho]

I think I worked it out. DefaultFolder X. Quit DefaultFolder X and all the problems disappear. Launch DefaultFolder X and all the problems coma back again. I've sent the developer a note to see if there's a way around this as DefaultFolder X is a crucial app for me.