After my bios splash, it shows „welcome to grub“ and then switches to the debian start menu for 3 seconds or so, then shows some terminal stuff and then starts kde splash and then login.
Yeah, the reason for this is that sometimes Debian doesn’t enable Plymouth splash screens by default, so you just see the text stuff. It actually annoys me a bit.
Not on my computer at the moment, so I can’t remember the exact packages you might need, but if I recall, they should be plymouth-themes and kde-config-plymouth (so that you can choose the splash screen theme in your system settings). You can also find other themes online, but I forgot the name of that website where all the stuff is. Pling? I think it’s that.
Anyway, once you have the themes installed, you need to sudo edit /etc/default/grub and append “quiet splash” (with the quotes) to GRUB_CMDLINE_LINUX_DEFAULT= (“quiet” might already be there).
You can also change the value of GRUB_TIMEOUT= in that file to whatever your preference might be for the duration of grub’s boot menu, but there might be other things you need to adjust in order to hide it completely and still be able to access it if necessary.
After that, run sudo update-grub so that it’s using the new config and choose whichever theme you want in the system settings.
Alternatively, grub-customizer is a GUI app that you can install to do all of the above (which will also update grub when you save your changes). Just don’t touch anything that’s not relevant. Stick to just the duration of the grub boot menu and add the splash parameter. Ignore boot priority, etc.
It should feel less “slow” to start up once all that’s sorted.