SUMMARY
The panel and applet/widget popups like the application launcher and tray menu still have their background blurred even when panel is set to opaque or adaptive (and a window is maximized). This can be observed during animations that involve fading in/out (opening widget popups) or manually decreasing panel/popup opacity (eg. with Meta+Scroll set to Change opacity in window action settings).

I don't know what optimizations are set in place by KWin for blur but this sounds like unnecessary GPU usage if the blur is still being calculated but not being useful because the surfaces are fully opaque.

STEPS TO REPRODUCE
1. Show Panel Configuration > Opacity > Adaptive/Opaque
2. Make sure panel is in opaque mode (Maximize a window if set to Adaptive)
3. Hover over widgets or open them to trigger some fade-in animations
    Or alternatively, Meta+Scroll down on a panel or widget popup

OBSERVED RESULT
You can notice blur is still being applied behind the panel and its popups implying blur is always active even when visually unnecessary or irrelevant.

EXPECTED RESULT
When the panel and its popups are opaque, disable background unnecessary blur.

SOFTWARE/OS VERSIONS
KDE Plasma Version: 6.4.3
KDE Frameworks Version: 6.16.0
Qt Version: 6.9.1

ADDITIONAL INFORMATION
May worsen performance or battery life on systems with integrated GPUs or on battery power, especially if users chose to make the panel opaque just for this reason while still keeping the global blur effect enabled for other more meaningful windows.