Android 5.1 vs Android 6: Memory Usage
These days most flagship phones have 3-4GB of RAM, so memory usage isn't a huge concern. However, for budget phones and Android One, memory usage will be very important. There's no Android 5 screenshot, as this is new to Marshmallow. It also slightly changed from when we first saw it in the preview builds of Android M.
With Android M there's now a dedicated RAM Manager tool inside of the apps menu simply called "Memory". This won't really matter for average users, but hardware enthusiasts or developers can use this section to see what's slowing down a device, what's using the maximum or minimum amount of memory at any given time, and more. It isn't anything major, but nice to see included.
Head into settings > apps > tap the three dots up top and select Advanced > Memory, and poke around with all the RAM manager information and tools. This will allow us to track down apps that are performing slowly, hogging resources, or even crashing the device and making it unstable. This may, or may not be in the final release.