This is not actually a bug (well the slow ai turns might be) this is designed more for multiplayer turns to keep the game going
the goal is simultanous plannig sequential movement
The concept is you can do any planning on your turn
i want this unit to go here on my turn
i want to build this unit/improvement on my turn
all the decisions you made on your last turn should stay in effect until you actually start your turn
for example if you choose to build a research centre on your turn lets say it costs 100 M and you produce 10M per turn then click end turn
now the ai is going and you think you know what i want to rush buy that research centre so you click buy
At the start of your next turn you will produce 10M and will then get charged 90*15 ( i think) for the rush buy
And on your following turn you will get your research centre