System slow and apps not working on Lineage OS 14.1
I am facing similar issues on OnePlus 2, but I have tried custom kernels which caused battery life issues. I tried to modify the cpu settings with Kernel Auditor (from google play, requires at least root). It did help a lot but somehow the app intersects with wifi hotspot, so it wasn't a solution.

Today I found a risk free option worth trying:
http://www.androidbeat.com/2017/03/how-to-stop-android-apps-killed-marshmallow-nougat/

Hope this helps