If you are an intruder come associate of the Ramsays, and you feel confident enough to enter their house abduct and murder JonBenet. Then its possible that you may feel just as confident about hanging about to write the ransom note.
Also if the perp is forensically aware, then using materials from the ramsay household is perfect, so it depends on how clever you think the perp might be.
Also the note may have been already composed, brought along with some other murder-kit items, using a "borowed" pad and pen, then for reasons unknown, it was re-edited resulting in what was left behind, so possibly it never took as long as suggested.
Then although it looks a serious ransom note, you have to consider to what extent it was part of staging, that is just one component in a staged homicide, all done to cause you to run like a hare down the kidnap and ransom track?
Assuming this could be the case then you have to ask, when was it penned , at what part of the staging sequence , currently most people think the ransom note was penned first. But this does not have to be, it could have come say last, and deliberately designed to buy time and confuse investigators. Since why kidnap then leave the body, any intruder or ramsay associate who entered and left the ramsay house that morning, was at liberty to take JonBenet with him. Away from the house nobody knows her status , dead or alive, so in financial terms there is an incentive to remove JonBenet's body. This was not done despite the perp LEAVING?
So its possible that the ransom note is not really just that , its really a deliberate diversionary device, possibly done towards the end of a staging process with the intention of buying time, so the stager can get away, cover their tracks, just in case JonBenet's body is discovered seconds after they leave the house. They need time to get back home and do a cover up and the ransom note may have been designed to supply just that?