unity8 response became very slow after placed for a long time (all night)

[Evan Wang]

$ system-image-cli -i
current build number: 28
device name: krillin
channel: ubuntu-touch/rc/bq-aquaris.en
last update: 2015-10-19 07:39:42
version version: 28
version ubuntu: 20151015
version device: 20150821-736d127
version custom: 20150925-901-35-40-vivid

This happens on latest images of OTA7 and OTA6+ both

Steps to reproduce:
1. Power on device and leave it there without touch for a long time (such as overnight and make sure have enough power)
2. Then check it the next morning

Actual results:
Device responses are very slow when unlocking or opening/switching apps, it will become normal after reboot