Why Does My Zenfone Use So Much RAM?

Welcome to the forum.
First off, stop trying to kill ram usage and here's why. Android, by nature, wants to fill ram up so when you start up things they're already in ram and give a snappier start up. When you kill ram with apps such a Clean Master, your phone will slow down partially because Android is trying to fill the ram back up plus those task killers often kill more than just apps and kill off services or apps that your phone needs running to be smooth and not become laggy. If Android needs more ram it'll close out stuff that is not being used without killing off services or other under the hood things that can cause the phone to act up.
Don't use Task Killers of any kind for best performance. CM is fine for residual junk cleaning and stuff like that but there really is no need to use it as a "ram booster" as that area is called in it.
Hope that helps.

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Is he running Greenify ? In the same apps as you? I used to use Greenify but it slowed too much stuff down for me .
I'm not familiar with the processors on both phones but if one is running faster than the other doing the same task I don't think that's a ram issue but a processor issue, especially since you have twice the ram as he. An app such as Watch Dog Lite on each device will show what is processing on each and the percentage of processing power on each. This is usually better to use when researching battery drain issues since rogue cpu usage causes unwanted battery drain but in your case it could be used to see if yours is running something in the background that his isn't, robbing you of processing power.
Most of those in your screen shots are running background do though in ram they are using 0 cpu, thus not slowing anything down or using any battery. They're just idle.
If your phones have Power Saver mode make sure both are set to off. One aspect of PS on many devices is it slows down processing speed to save battery

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Try uninstall Greenify as well and see if it speeds up. If so that's the culprit, if not just install it back. Again as stated above, we're looking for apples to apples to nail down the cause.
Yes that is normal for Android. Try not to think like Windows behaves because when Windows fills up it doesn't dump ram when more is needed. Windows just slows down until things are manually closed out whereas Android (and ios to a certain extent ) does the closing as needed in the background.

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