Comment 0 for bug 1479250

top reports the unity8 process is consuming about 1% CPU even when the phone is idle (screen on):

11688 phablet 20 0 482184 101352 44872 S 1.0 5.4 0:37.10 unity8

This makes unity8 the top consumer of CPU. 1% on modern CPUs like you find in phones is actually a very large amount of work going on (~10 million clock ticks spent executing instructions per second on a 1GHz core).

$ system-image-cli -i
current build number: 272
device name: mako
channel: ubuntu-touch/devel-proposed/ubuntu
last update: 2015-07-29 05:53:01
version version: 272
version ubuntu: 20150729
version device: 20150708
version custom: 20150729

Running eventstat shows this CPU time is being used in 5 events per second:

 Event/s PID Task Init Function Callback
   36.36 0 [swapper/0] hrtimer_start_range_ns tick_sched_timer
    5.05 11688 unity8 hrtimer_start_range_ns hrtimer_wakeup

So it sounds like QtMir's frame dropper is the culprint.