That could be seen as premature optimization. If you don't know things like the average number of cards, or the average for the power users then you don't know

That could be seen as premature optimization. If you don't know things like the average number of cards, or the average for the power users then you don't know if you will have that problem. That would be like saying they should spend time on supporting 30,000 (made up number) cards right now.