I have tried replicate it but was unsuccessful. It might have been fixed. It was quite sporadic before though so it might not...
We are having the same problem on our server, though it has nothing to do with the sieges. Every night our server peaks real bad which cause block lag. Whenever we kill off the zombies or they die at daybreak the server cools down and the block lag disappear.
Edit: We're on 1.6.1
I tried it again on 1.6.2 and it still there.