Data off when screen off is fine.
Yes, all of your assumptions are wrong. They are wrong on Android, they are wrong on Windows for PC, and they are wrong on Linux as well.
1. RAM has nothing to do with internet connectivity and speed. In terms of execution of tasks, freeing it up makes it slower. For one thing, the used up RAM is used by preloaded apps. For example you always use Chrome. Hence it was already preloaded to RAM so that when you press the icon it is already loaded up. Preloaded apps on RAM hardly uses any battery. Empty RAM and full RAM uses same amount of battery. However, if you cleaned up RAM, when you open Chrome, the CPU will have to run to call Chrome from storage, and load it up to RAM first before opening. So it uses up more battery to load the app to RAM, when it should have already been loaded if you did not clear it.
2. The higher then numerical value of the RAM means the phone can be faster because you can use more RAM to load up more apps to RAM.
3. Turning off background apps via task killer will not make the phone faster because once MacroDroid automatically kills the task, the Android OS will automatically start them again because the apps are coded to be running in the background. For example Twitter. You kill the task to supposedly make the phone faster. However, you set the app to sync every hour. So one hour later, the app will just turn on again, using up more battery to start up and reconnect to internet, when it wad already connected and dozing.
3. The more you stop background apps, the more the OS will be confused. The app was designed to be running, but it was turned off. So the OS will turn it on again. Then the task killer will turn it off, then the OS will turn it on again. So this uses up battery, and makes the phone slow down because the OS is confused because it's trying to turn on again the apps you closed and you're asking it to do other stuff as well when using it.
4. Apps running in the background are there because they were set to do that. Viber is running in the background because it's polling for new messages. If you kill it, you won't receive messages. So the OS will turn it on again. As said above, the more you stop running apps, the slower the phone will become because it will become confused.
5. Apps running in the background do not use data unless they are supposed to. Killing them will make it use more data. For example, Twitter. If set to sync only once every hour. So for example it will only use data at 1pm and 2pm. However if you kill it at 1pm and the OS starts it again at 1.20pm, it will use data at 1.20pm. Then you kill it again at 1.30pm, then starts again at 1.40pm, it will use data again. It should have only used data twice, but you made it use it 3x.
To keep the phone running normally, forget about it. Do not think of how much RAM it is using, or how many apps are running in the background. Just leave it alone and let the OS manage it. And do not try to do too much. If you have a cheaper low end phone, do not expect it to be fast or work as good as a more expensive phone. It doesn't matter if it is octacore or not. The US version of the Galaxy S7 is a Snapdragon quadcore but it is faster than an octacore S5. Basically, if your phone is not as expensive, it will be slower. Do not expect too much.
For decreasing data usage. Turn off sync for any app you do not want to be always using data. Set the apps to only update when connected to wifi.
