The siege of the city is a really long process and the often captured city is in extremely poor condition. I would suggest increasing the chance for buildings to be captured from 66% to 75% or 80%. It's not fun to capture a city on 300-350 turn and then spend 50 turns, only to restore its production and scientific capacities, defensive potential, even buying every building that requires more than 1 turn for buildings. Especially in the case of capturing city-states - it takes a lot of turns to raise a city to a good level.