[T-Mobile Galaxy Note 4 Q&A, Help & Troubleshooting] duh-rel:
Stock Ran Better, But Still Unusable As DD

I've had similar issues with my refurbished white Note 4 (~14 months old). It's been running FireKat w/Xposed for 90% of its life, but a month ago I began experiencing slow performance with apps freezing. I decided to make the jump to Marshmallow and clean flashed stock EPG2. Things got worse with the freezing apps and slow performance (e.g. touch input of PIN lock was delayed and it'd take awhile to unlock to the home screen).

I tried flashing Maximum OvrDriVE ROMs and still no good. Tried both EPG2 and EPE3 and a couple different updates (didn't try the latest Maximum OvrDriVE MM v3.2 Update).

I tried flashing the PIT, as recommended in another thread, and still no luck.

Somewhere in between flashing stock MM and Maximum, I experienced the mmc_read error and also would have to resort to battery pulls and de-energizing the device (holding the power button with the battery out and not attached to a charger), before it could power on again. Sometimes it would power onto the mmc_read error screen, sometimes hang on one of the splash windows (usually Android or Samsung, sometimes T-Mobile), and occasionally just shutdown in the process (typically requiring another battery pull and de-energizing).

I tried everything from Odin and TWRP to flash ROMs, to Samsung Kies to factory restore, to Smart Switch which never worked saying that my device was unidentified (or something to that extent).

I've tried a mix of leaving the SD and SIM cards mounted, but didn't see a difference.

It's been on the original battery which provided about 3 hrs of SoT prior to these issues. I didn't notice anything correlating to the battery level, although I rarely have the battery level below 15%.

I don't think the problems correlate to temperature either. After flashing, with the phone at temps above 110°F, sometimes it can run a few mins with no problems, other times immediate reboots, and everything in between. This happens when the phone has cooled off too.

I gave up last week, and relegated my SIM to a Nexus 5. I left my fully charged Note 4 on stock EPG2 (no add'l apps installed, no accts added) sitting a few days in temps from 60°F to 95°F and didn't notice it randomly rebooting. When I turn the screen on, sometimes the apps don't hang, but most times they do. I decided to give it another spin. After a week with no use, besides turning the screen on to check the battery level and charging via cable, I put in my SIM and began using the phone again. It worked for a few hours of extremely light usage (a couple calls and a few texts) with little issue (some slowing), but the random reboots and slow performance emerged once again (hadn't seen the mmc_read error anymore).

I think the issues are tied to hardware failure. I don't notice the rate of app hang ups, slow performance, or random reboots tied to the device's temperature. They happen regardless if the phone was cold or hot and regardless if the phone was plugged in charging or running off my 2 y.o. OEM Samsung battery.

Other things to mention is that in addition to the factory Samsung charger/cable, I have routinely charge the device using an HP Touchpad charger/cable, and for the past ~7 months used an Aukey car charger/cable (Quick Charge 2.0 AUKEY Dual Port Car Charger). The device has been charged using a few other methods (LG charger, from my ASUS laptop, my Dell desktop), but very rarely. Maybe the use of other chargers contributed to an accelerated reduction in hardware life (perhaps in combination of using custom ROMs)?

I've sent it to Samsung for repairs and will update you on what I hear back. In the meantime, being phoneless feels odd (my son dropped the Nexus 5 and cracked the screen).