Select a large number of messages in the Inbox, say 50 or 100. Try to drag them to a folder. Notice that the spinning beachball is active as soon as you start to move them. Keep the mouse button held down until the beachball turns into the <+ pointer. Note the time. I just dragged 127 messages and it took about TWO MINUTES for the pointer to change. This performance seems to scale with the number of messages being dragged, one message is almost instantaneous, but as you select more and more messages it gets worse and worse.
I have a 2GHz MacBook Core Duo with 2G of RAM and not much else running other than Notes, running OS X 10.5.3.