screen lock

I am using FreeBSD 10.3 with slim and MATE.
Is there a way to lock the screen after 15 min of inactivity and show the password prompt?

that seem to be resource intensive. i am OK with a plain black screen or slim log on prompt if i could log out the user after 15 min of inactivity
You can disable the screensavers entirely if you run xscreensaver-demo