I have a Dell inspiron 14, and when I use the keyboard hotkeys Fn + F12 or Fn + F11 to adjust the screen brightness, the system slows down and the brightness is adjusted slowly.
Sometimes it keeps adjusting in a loop, even it reaches the minimum or maximum. When it stops adjusting, the system backs to normal, but sometimes the keyboard freezes and I have to reboot.
When I was using the 16.04 version of Ubuntu, I wasn't having this problem. The problem doesn't occur when adjusting the brightness thought the top menu, and when I'm not logged in some user account.