Andriod 5.1 remembers running apps on reboot

[AC Question]

I have updated Sony Xperia Z2 to Android 5.1.1 and noticed that now, if I reboot the phone, it restores apps which were running before reboot.

How do I disable that?

I cannot find any setting or even any information about it. If there are a lot of apps running, I get a message during startup "Restoring apps" which delays startup time.