Emergency shutdown leaves the Desktop on for much longer in Stretch

With Tails3.0beta1, beta2 and beta3 the emergency shutdown is very slow with a Toshiba Satellite:

Steps to reproduce:

- Start Tails 3.0beta3
- Take out the USB stick

What happens:

- The Desktop stays there, untouched, for 8 seconds. Then starts to corrupt for some 5 seconds more, then finally a black screen and a blinking CapsLock key.
I need to manually press the power button to make it shut down.

What should happen:

Before 3.0, the screen went black in maybe 2-3 seconds.

I can test other ISOs on this hardware.

Related issues

Related to #12354 (closed)

Original created by @emmapeel on 12393 (Redmine)

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