about system does not work

[smoreau]

That looks really ok to me and really fast. So the issue must be in sddm or plasma starting itself.
Can you take a look if there are any errors in /var/log/sddm.log

So execute for example

Code:

cat /var/log/sddm.log

to get an output. If nothing is displayes it might be empty which then means it really is a plasma startup problem.

In that case can you try creating a new user and see if it's login times are faster?

So as you expected sddm.log is empty. A new user has a similar login time.
What I can tell is that when on this same machine I updated 17.03 to plasma 5.9.5 all was OK for the boot time. So for me plasma 5.10.1 seems implicated.