Shutdown query window takes too long to pop up

I got this problem since ubuntu 17.10, and now it's still there in ubuntu 18.04. That is when I turn off my computer, the window which says "The system will power off automatically in 60 seconds" takes over 30 seconds to appear. It's used to appear instantly.
It's not that I got this problem every single time I turned off my computer, but the chance is quite high, about 70-80%.
Although I can just open terminal and type poweroff to shutdown instantly but it still annoys me.