What I do now

I read a book about frontend performance, then RAIL appeared

What I don't understand

RAIL is based on initial letter of these four words
Response, Animation, Idle, Load

About Idle

Javascript should finish action less than 50ms in idle time

so I think it means like that

finish loading contents
↓
but javascript still needs to do something like "getting data from twitter API", "lazy load image"
↓
each action should finish less than 50ms ?

↑But it is absolutely impossible because for example getting data from some API should less than 50ms is like magic.

I think I misunderstand something, hope someone explains me "What action should be less than 50ms in idle time of Javascript"