I was monitoring my syslog while pressing the button.

Apr 29 14:37:35 ubuntu-phablet powerd[949]: handle_requestSysState from :1.13 - 
ACTIVE (1)
Apr 29 14:37:35 ubuntu-phablet powerd[949]: name_watch_add: looking for :1.13
Apr 29 14:37:35 ubuntu-phablet powerd[949]: watching :1.13 to see when it 
disappears on dbus
Apr 29 14:37:35 ubuntu-phablet powerd[949]: libsuspend: acquire_wake_lock: 
powerd_power_request
Apr 29 14:37:35 ubuntu-phablet powerd[949]: handle_requestSysState - SUCCESS
Apr 29 14:37:35 ubuntu-phablet powerd[949]: Enqueue state change to ACTIVE
Apr 29 14:37:35 ubuntu-phablet powerd[949]: exiting suspend
Apr 29 14:37:35 ubuntu-phablet powerd[949]: libsuspend: exit_suspend.
Apr 29 14:37:35 ubuntu-phablet kernel: 
[21759.723201][Ker_PM][request_suspend_state]wakeup (3->0) at 21760134073325 
(2015-04-29 12:37:35.882532317 UTC)

< exactly here there is a short delay>

Apr 29 14:37:36 ubuntu-phablet powerd[949]: Emitting signal for transition to 
state ACTIVE (1)
Apr 29 14:37:36 ubuntu-phablet powerd[949]: Transition to ACTIVE complete
Apr 29 14:37:36 ubuntu-phablet powerd[949]: libsuspend: release_wake_lock: 
powerd_power_request
Apr 29 14:37:36 ubuntu-phablet kernel: [21760.273491]mtk-tpd: TPD wake up
Apr 29 14:37:36 ubuntu-phablet powerd[949]: light_dev: setting brightness to 120
Apr 29 14:37:36 ubuntu-phablet kernel: [21760.310044]mtk-tpd: TPD wake up done
Apr 29 14:37:36 ubuntu-phablet kernel: [21760.310066]<MAGNETIC>  
mag_context_obj ok------->hwm_obj->early_suspend=0 
Apr 29 14:37:36 ubuntu-phablet powerd[949]: we get signal from :1.13: 
DisplayPowerStateChange
Apr 29 14:37:36 ubuntu-phablet powerd[949]: Received DisplayPowerStateChange: 
state=1 flags=2


Since some of those messages are from the kernel ringbuffer, I looked at the 
corresponding dmesg output:

[21606.412808] [Ker_PM][request_suspend_state]wakeup (3->0) at 21606823677239 
(2015-04-29 12:35:02.572136231 UTC)
[21606.953480] mtk-tpd: TPD wake up
[21606.980027] mtk-tpd: TPD wake up done

So there seems to be at least half a second of delay between the kernel
registering the button press and the display actually being requested to
power on. Since display power-on is not instant, the total time from
button press to visible display content is close to a second on my
device in best case, sometimes up to three seconds and in rare cases the
display does not turn on at all.

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https://bugs.launchpad.net/bugs/1421455

Title:
  Slow wake up time on physical power button pressed

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