When installing/using a high DPI screen (e.g. emergent QHD+ standard ~235DPI) KDE visuals (a) default to settings that are near unusable because everything is so small and (b) there is no easy way to reach settings that are usable i.e. to get to a usable state, do:

KDE System Settings (Configure Desktop);

in Application Appearance, go to Fonts, enable Force Fonts DPI and set it to 144 or above;

in Icons > Advanced set every icon type to at least one higher than it starts at;

in Workspace Appearance > Window Decorations > Configure Decoration set Button Size to Very Large;

set the Panel height to be 40+.

A new KDE user would struggle to achieve a usable state. Even if detecting the DPI is not possible (DPI config for X issue?) there should be a visual setting option for setting it to some levels (e.g. normal, high, very high), specifying it as an integer or a slider which then sets the above or makes for an equivalence.

Additionally the system tray icons do not scale. There's an old bug on that, opened in 2006.

From a UX experience this should be seen as a "Grave" severity level (to a normal user the software would be seen as basically unusable) but have left marked as Normal to avoid a smackdown from a "there's a workaround" posse member. It's not really a workaround for a normal user, too many steps in too many places.

Reproducible: Always

Steps to Reproduce:
1. Install KDE distro on device with high DPI screen;
2. boot up and log in;
3. squint.
Actual Results:  
when logging in the first time the default text sizes, icons, window decorations etc. are too small to be usable, 

Expected Results:  
Post logging in should be able to gaze in wonder at awesomely pretty screen and vast screen estate without anything detracting from the experience ;)