or he should watch at the build.prop or there is something edited or need to wipe data from google playstore or there is some xposed module which is causing it I don't know but if you edited a lot on that custom rom with kernel and like that backup it first don't go immediately to stock or else it was just wasted time for you. It could also be your dpi but I don't think that's it also wipe cache in recovery that might work.
Devices i owned
Htc Elf-Htc Google Nexus One-LG Optimus 2X-Samsung Galaxy S2-LG Nexus4

My status:
HDD Half working :/ Ubuntu went black. Windows first 60FPS with Titanfall now 20-30 with laggspikes to 0FPS Time for a replacement soon..
Phoneless nothing to customize now so bored...

HIT the button