It was my impression that RF was designed for casuals unable to commit massive amounts of time to raiding.
so far, the first wing last week was over an hour for 3 bosses. this week, my LFR group for forgotten depths spent an hour and a half getting to the last boss and is currently still wiping on him. i had to leave because i ran out of time.
so with an average of lets say 30 - 45 minutes per boss kill in LFR, we're looking at potentially spending more than 10 hours per week running the new raid just in LFR.
add the 12 to 20 hours most raiding guilds spend on progression content, you're looking at 30+ hours of raiding per week. there are not enough bosses for this amount of time expenditure and its just too much time anyway.