Implementing Fast App Resume for WP7/8?

So, fast app resume for apps was introduced in WP8 yet in over a year only like half of apps made for WP8 support it. Lets not even start how I don't think when Skype supports it.

So developers loved Fast App Switching(but took their time adopting that as a standard too) and are now more or less forced by the new WP8.1 guidelines to implement FAR into their apps.

My question, as a young developer is: Is it so hard to implement it, since it improves the experience considerably ?

PS: I'm not completely sure if FAR it's available on WP7 too