I can lower the settings for the game itself (e.g. for when you’re using Play in Editor) but I can’t figure out where to lower the graphics settings for when I’m in the editor. Really need this because my computer is a potato 
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You can do so by going to Settings → Engine Scalability Settings and turning down most of those settings. Some don’t apply in-editor but the ones that can will. This Settings button is located on the main editor toolbar.
You can also disable real time thumbnails in the content browser by clicking the “Options…” button.
Might also want to disable the real-time viewport by clicking the downward arrow in the viewport and unchecking Realtime.
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I should add… If you’re like me and not very smart and spent 15 minutes trying to find the Settings Button after looking through Editor and Project Settings…
The Settings button in this specific case is the button located along the top bar (by default) between the Marketplace and Blueprints button. Gear icon.
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Adding to answer.
At the top of the engine (where the marketplace icon is) you will see an icon called “Setting”, press it, then look for Engine Scalability Settings, and press Low, but I do not 100% recommend the low settings as it makes vertices look terrible, but I still use it, but I might change
This is an interesting question that hasn’t really be solved yet properly - or more like I haven’t found a real answer yet.
Most of the answers mention the engine scalability settings but those only govern the play mode.
At the same time, the editor did lower my resolution etc. brutally when it realized the rig I was using was way too weak - this affected both the play mode AND the preview which you see during editing.
Sadly it didn’t do that ever again so now, at the same weaker rig, I can’t manually reproduce that lovely low-res preview.
It’s really weird. Maybe I can try setting graphics in world construction blueprint or similar? 
Thank you very much for your reply! So far, I’m new to setting up graphics for a game. But I’m going to work on it tomorrow. I will try to apply your advice.
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and try restart engine 
Yes, this is it, perfect man appreciate