'System UI isn't responding' means the interface layer itself, home screen, status bar, stalled. Usual suspects: a misbehaving widget, a third-party launcher or an outdated WebView.
- 1Tap 'Wait', then remove recently added widgets and live wallpapers.
- 2Update 'Android System WebView' and Chrome in the Play Store.
- 3Using a third-party launcher? Switch to the stock one to test.
- 4Clear System UI cache: Settings → Apps → show system → System UI → Storage → Clear cache.
- 5Wipe cache partition in Recovery if it keeps happening.
- 6Low RAM devices: reduce home screens, widgets and animations.
Tip: System UI crashes right after a system update usually settle after a couple of days of re-optimization, wipe the cache partition to accelerate that.