I got my new monitor today and replaced my main/old one for it, checked it out, all good. Then I set my old monitor to the side, plugged it in as secondary and got this problem.
See the attached image. It's important to note that only certain PARTS of the Windows UI are blurring. You can see three main things in the pic: the less sharp text (Windows Explorer - looks even worse on screen), the more sharp text (in the web browser) and a copy/paste of the main display's text for comparison.
So it feels that something isn't resizing properly. If I load up Photoshop, it isn't just text that's blurred, it's the whole UI. Likewise Explorer.
I have had this issue before (upon plugging in a new monitor) and a good old fashioned restart sorted it out. Not so this time.