Very slow booting from unpowered but fast rebooting


I installed Ubuntu Mate on my new Probook 440 G4 (Ubuntu one half of the SSD - other half still unallocated - second hdd still unallocated) and found some strange behaviour.
No special problems at installing, vanilla system except changed colors, installed geany and some other minor programs.

When I start the notebook from "power off" the GUI elements take extreme long to boot up correctly (>3min.) and sometimes the notebook also hangs seemingly random (e.g. at "stopped sesseion c2 of user" and after "save/restore soundcard state" or "shutdown point reached - or something similar" The problem occurs after login, the wallpaper and the taskbars are already loaded, but the bars seem to be missing the theme.
I also can't open the terminal or anything else until quite exactly the time the theme also loaded, when it does everything seems quite responsive and the things I started while it still was loading all pop up instantly.

Changing the theme did not help.

On the other hand rebooting just works fine and I never had a problem with that.

systemd-analyze shows nothing special and that the system is booted within 20-25 seconds.

I haven't found anything similar by, here are some screens of how it looks:

Edit: The mouse is still responsive and has the spin animation while hovering over the wallpaper, but the is no reaction to the input.

Edit2: After Restarting some times the system now doesn't finish loading at all sometimes.

Thanks in advance for trying to help me out.
Greez, Lukas