I don't think this is a bug, rather a topic for discussion. The length of time it *should* take to claim something is a matter of opinion & game balance. I resurrected DK2 a couple of months ago, have been playing a lot recently and this stood out for me when playing the beta. Of course I may be wrong - I didn't actually time it - maybe I'm just an impatient keeper or all my imps are Level 10 ;-)
I think the balance and behaviours of the Imps is probably one of the most critical elements in the gameplay
Yes, I found bridges switched sides very quickly too!
I think it should take sufficiently long to take something so that always a fight might occur over something. If you can capture something in a matter of seconds there would not be much room for a battle over that object.