I’ve just realised why Dolphin is slow in loading directories, it’s because “Previews” are ON globally. There’s only one directory where I would ever want “Previews” to be ON and I can’t figure out how to turn “Previews” OFF globally. Anyone know how to do this, thanks.
How about Configure Dolphin → General → Previews and then unticking everything? Does that work?
Thanks but I’d already tried that and it didn’t work.
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Found it! 
View -> Adjust View Properties:
Un-tick “Show preview”
Click “All folders”
Tick “Use these view properties as default”