yeah, I hear ya. I kinda figured that the lens would be more for slow moving or static animals and birds. I highly doubt it would keep up with a BIF.
If there are switches like the long and short zooms on the 70-200 or 100-400s then it might be alright as it won't be hunting as much.
it would also be a monster to tote around.
as demonstrated by the 400mm beside it here
but I suspect if I were using this lens it would be in conservation areas where the aminals/birds might be in the middle of water or from across the pond and etc.