Ok. This is quite technical and I’m not sure I understand it all but if you’re having trouble reading the screen on your external monitor attached to your Mac laptop or Mac Mini/Studio this could be it.
I think the gist is:
If you use software that needs a lot of power from your Macs graphic card (like video or 3D rendering or photo editing) this might slow down your Mac. Additionally, text may look blurry. Why? It’s all to do with rendering your Macs native screen size to your monitor’s.
Your monitor needs to be true 4K (4096 pixels) not Ultra HD (3840px confusingly sold as 4K). You could solve the problem by buying a 1440px monitor – this would solve the slow running – but it’s not ideal.
From the first video, I’ve listed some references from the comments as well as references within these.