Please create a support ticket about this so we can get a sense of your query times. The number of hands you have in your database should have zero effect on how fast the hud populates.
PokerStars writes all of the completely hand histories to disk in the order that they were dealt. Thus the only thing you need to do is to leave the table window open for a few minutes after you sit-out so it can finish writing the last few hands.
Does that mean that one of them is not being used any more? All of the columns formerly in _hand_player_detail are now in _hand_player_statistics. Your custom stats will be updated automatically. I'll try and get a solid answer about the session splitting feature. For now everything is still hard c...