Windows 10 startup suddently extremely slow (especially explorer and filesystem related stuff)

lose lots of its speed expecially on the startup. At first i was able to fixit just by killing the explorer right after logging in and just restarting it.

This is very likely a context error. This is how File Explorer and Task Manager (and some others) behave when context handling goes wrong. It can be very difficult to find a specific context error because there are so many context handlers.

Try the following approach (I have used this before).

(1) Make a new, test Windows User Profile (Account). Log into the new Account and see if the hanging behavior persists.

If so, copy the data, favorites, email and documents to a neutral location (say, c:\temp). Make a new user profile and recover your data.

If not, then:

(2) Run a Windows 10 Repair Install.

https://www.microsoft.com/en-us/software-download/windows10

Windows 10 is running, so click on the Download button (not Upgrade Button) and select Run.

Use the option to Keep Everything to begin with.