My question is easy.
I remember a function in an older version of Photoshop where I could say Photoshop to preview things I move around in lower quality to improve performance.
Where can I activate it in Photoshop CC? Or doesn't it exist anymore?
I've been using Photoshop for a long time and there's still features I don't know about or don't know well so this is certainly possible but I really don't think you're right. I'm guessing you're thinking of InDesign which has Display Performance:
In Photoshop you can do other things to improve performance such as Cache Options and Graphic Card settings and such. Two things you might consider are:
Turning off animated zoom: Open Photoshop → Preferences → General → Animated Zoom & Enable Flick Panning → “Unchecked”
Turn off Layer Thumbnails: Open Photoshop → Layers Panel → Layers Panel Menu (dropdown) → Panel Options → “None”
Since the thumbnails have to constantly update as well this can really improve performance. But the sacrifice is no layer preview. Gotta be your choice. For more ways here's a good link:
How To Optimize Photoshop For Better Performance by David McKinney