Besides, Windows is very busy on allocating resources, and all kinds of
auto-start service, applications are running behind including AntiVirus and
Firewall programs, and it could also be checking its server to see if there
are new updates and if your virus definitions are outdated, while loading
its own services and competing with others for resources.
With all these things added up, it may take some time for it to be totally
ready.
There are some articles on the net about how to fine tune (or tweak) your
system (including disable unnecessary auto-start programs, auto update
features, etc.), you may wish to Google for more information.
Just a personal preference, I tend to disable all automatic update features
for all applications including Windows, so I could have more control on when
and how to install updates without any surprises such as you need to restart
your system or you need to close "this" application to install updates/
But I do regularly run all updates manually.