Encrypt whole system?

My laptop isn't under my supervision most of the time. And I'd hate it if someone were to steal my SSD, or whole laptop even, when I'm not around. Is there a way to encrypt everything, but still keep the device in sleep, and unclock it without much delay. It's a very slow laptop. So decryption on login isn't viable, takes too long. While booting up also takes forever, so it needs to be in a "safe" state when simply logged out. Maybe a way that's decrypt-on-demand?

I'm on Arch with KDE.