I've been using CyanogenMod (version 12.1-20150901) on my Nexus 5 for around a year and things were running perfectly until last month.
My data plan had been almost completely used up after a couple days. At the beginning I blamed it on activating my phone's wi-fi hotspot feature and downloading some files on the computer through it. A couple of days ago my data plan reseted, so I decided to activate the internet and **in less than 12 hours, my phone had consumed +400MB!!!**

Taking a closer look, the **Android OS had been used more than 10GB** of wi-fi over the last month (it seems to have started around the 8th of May)

[![Android OS High Internet Consumption][1]][1]

So as some android forums suggested, I decided to install "Onavo Count" to find the culprit. It turned out to be Google Apps/Google Play Services:

[![Google Play Services High Internet Consumption][2]][2]

It registered that Google Play Services had a 189MB data consumption just after a couple of hours I had Onavo installed! I turned off the 3G immediately, but still after 3 days, Onavo registered it consumed 1.8 GB of Wi-Fi...

I've looked through many forums, yet none has been able to provide a solution that works for me.

 - Some suggest one should restrict the app's background data, but that doesn't solve the high Wi-Fi consumption problem...
 - Others suggest upgrading CyanogenMod to the lastest stable build (v 13.0-20160418), but that broke completely the functionality of all Google Apps (constant crashes that make the phone unusable... luckily I did make a backup... backups are good).
 - There are suggestions of removing permissions and functionality, but I've played with every setting and removing all Google Play services functionality to no avail...

Has anybody encountered this problem and has been able to solve it?

  [1]: https://i.sstatic.net/aTfDE.jpg
  [2]: https://i.sstatic.net/te8va.jpg