Hi, using Plasma wayland, I found that the startup time for plasma desktop environment was about 25 seconds. Upon testing Plasma X11 session as opposed to wayland session, plasma X11 started in a few seconds. I searched and discovered a tip that if the splash screen is disabled, plasma wayland also starts in just a few seconds. and it worked! So, if you experience this, simply use plasma system settings → Colors and Themes → Splash Screen and select None (can also search for splash in the settings dialog).
https://bugs.kde.org/show_bug.cgi?id=482082
Hope helpful!