Comment 51 for bug 847001

Same issue here with Ubuntu 18.04 on Dell Inspiron 15R 5521.

I have tried changing
       GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
       to
       GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor".
on /etc/default/grub and did not help.

Also tried the solution on https://ubuntuforums.org/showthread.php?t=2208278 and did not do anything.
Both brightness and volume keys are really slow do adjust and the brightness ones always freeze the screen for good 5 seconds.