Battery

That depends on your current usage and apps installed. Apps are typically just revenue makers, so many are poorly written. There's absolutely no reason why most of them should lag, or remain, in memory when not in use. Some games even load or remain, with the sole purpose of pushing the occasional ad. Uninstall all apps that you don't use, or could use something else for. For example, do people really need a flashlight/torch app with ads when they simply use it for the function that's easily accessible in the notification area?

After you have uninstalled these force stop all apps under settings that shouldn't stay in memory. You shouldn't be able to accidentally force stop anything essential. Just start with literally all games, then almost all apps. If you use a launcher don't force stop that of course, not force stop Swiftkey if you use that. If you use Swiftkey (some themes are quite nice) you can force stop Gboard and the plugins, and likewise you can force stop launchers you don't use. Launchers are a preference thing, but just avoid those with ads (if you must use that one purchase it). I like Microsoft Launcher myself (no ads) with customisations like their icon pack, transparency with blur and opacity (basically Windows 7 style aero glass), daily wallpaper, alphabetical list for the app drawer etc.

Both Microsoft Launcher and Swiftkey are completely free, resource friendly, have beta programmes and are readily updated. 

Basically go minimalist and work your way up from there. Bad apps, particularly memory resident apps usually for the purpose of pushing ads, are both battery draining and can cause instability