[Sprint  Galaxy Note 4 Q&A, Help & Troubleshooting] samep:
You could likely do better with later Marshmallow releases and possibly custom ROMs if that's your thing. I'm on the PJ1 update running UN7 (Note7) port.

Between, Samsung, Google and Sprint, there's enough bloat on a stock Sprint Note 4 to sink a ship. And Samsung can't optimize the RAM caches well enough to suit individual preferences because it favors its native apps that are baked in as well as other system apps from Google.

This optimization may not favor the apps you prefer to run at will so you need to understand how to deal with it. There's Samsung's version of Doze and there's Android's version of Doze on your Marshmallow Touchwiz firmware. Samsung's version will routinely shut your apps down and doesn't do a good job of real time RAM management while sleeping. It's based on a 3 day run theory but it doesn't my user apps use at all; it shuts apps down that haven't been used in three days. This favors apps that it runs in the backgrounds continually.

So turn that of and focus on Android Doze is my advice. You can find both methods in battery management. Smart Manager may a shortcut to battery management but you can use Settings to find it too. You may see App power monitor on first battery screen; I advise turning that one off. Android doze can be found by tapping Battery usage then tapping 3 dot menu button in top right to goto Optimize battery usage. Once there, the dropdown menu will allow you optimize doze by showing all apps. I disable ones I don't want running continually but enable those that may be impacted by doze with screen off only. If you ever wonder why your music or downloads stop with screen off, here's a place to start. Push notifications should happen anyway with doze active so those may not need enabling to run continually.

You may consider freezing apps that show as running when you don't prefer them to even you're checking running apps. You can use Smart Manager often to optimize RAM or get an app that allows you white list some apps and dump cache for non white listed apps on a scheduled basis.

Personally I find Marshmallow Touchwiz poor for RAM management. The UN5 Lollipop ports managed RAM better without having doze. Somehow I think even the Note 5 may not have good RAM management on Marshmallow. Maybe Nougat is a different story? But I do know that Marshmallow proved to be a bigger challenge than Lollipop did, especially the ports. Why would I run Marshmallow? I guess its inherently faster than Lollipop. But only when optimized, IMO.

I also use Boost+ by HTC found in Google Play Store to free up RAM. Samsung thinks many apps I use regularly aren't being used. HTC Boost+ can discern app use better and logs freed RAM so you know it's working.

As a user, you can discern if these suggestions help or not. Just lag, preferred running apps and battery management advice. They may not be every user and YMMV.

Edit: pay close attention to battery capacity at reboots; bad batteries do have a tendency to shutdown, possibly reboot as well. Last time I had a bad battery, it was on a lollipop ROM; phone shutdown at 30% and plugging in charger showed 0%. Marshmallow may be different; battery is suspect for reboots in Marshmallow. I haven't experienced reboot issues with Marshmallow or have a bad battery to add to this suspicion though.