T580: setting brightness freezes desktop temporarily

[Veraellyunjie]

Thinkpad T580, no-nvidia version, only intel iGPU.
There are Fn+F5 and Fn+F6 keys to control screen brightness.
In Windows, they cause brightness to gradually decrease or increase.
In Xenocara, brightness switches immediately,
   yet desktop freezes for about a second.
In TTY, brightness switches immediately, and no freezes occur.
`xbacklight -steps 1 -set ...` also causes a temporary freeze.
The Fn brightness hotkeys don't trigger keypress events in `xev`.
Can freezing be fixed?