In case of two display with different DPI, Windows one as a base and scale on one of the screen. Scaling is not perfect and that make font blurred.
Issue known to Microsoft. See for example.
Office apps appear the wrong size or blurry on external monitors
Some desktop applications may appear blurred on high-DPI displays
No good solution at the moment.
Perfect solution for now is not to use different DPI monitors.
And not connect external monitor to notebooks.
