Open Radar

 
Summary: 
Dragging a Space around, so as to reorder it, when in Spaces' overview is marred by a very slow, unresponsive animation of the Space being dragged that lags several seconds behind the mouse cursor's movement.

Steps to Reproduce:
1. Enable Spaces in System Preferences > Exposé and Spaces preference pane.
2. Enter Spaces overview mode by launching Spaces.app in the /Applications/Utilities folder, a keystroke such as F8, or an assigned screen corner.
3. Click on the gradated blue background of any Space, whether with or without windows in it, and drag it to above another Space so as to swap the position of the two.  Attempt to do this at a fast dragging speed; for example, drag the Space in a circular motion across the entire screen.
4. End the drag by letting go of the dragged Space while positioned on top of the Space intended to swap places with the dragged Space.

Expected Results:
The Space being dragged animates smoothly, stays in sync with the mouse cursor movement onscreen, and does not significantly lag behind the mouse cursor movement while being dragged.  Upon ending the drag, the two Spaces immediately swap locations, animating smoothly between the two positions.  This behavior is consistent no matter how fast the Space is being dragged across the screen.

Actual Results:
The Space being dragged lurches in the direction of the mouse cursor's movement, regardless of its contents and regardless of the contents of the Space below it (that it would swap with).  Its animation is smooth, but often dauntingly lagged behind the mouse cursor movement, even at slow movement speeds.  The animation appears to lag a particularly great deal when the gradated blue background of the Space below the dragged Space dims, and dragging the Space over subsequent Spaces in the overview arrangement increases the lag.  Moving the dragged Space in a rapid circular movement pattern that crosses and dims the background of several Spaces underneath causes the dragged Space to lag behind the mouse cursor by several seconds.
Very rapidly dragging a Space over several Spaces and releasing the mouse button while the Spaces' background are still darkening can cause the transparent, dragged Space to appear on-screen even after the drag has completed (when the mouse button is released).  All these lag problems appear to worsen if a Space contains one or more windows, as opposed to no windows.

Regression:
To my knowledge, this lag and other issues described above were not present in any release of Mac OS 10.5 Leopard.

Notes:
I witnessed this performance bug on a MacBook Pro 15" (July 2009) that reproduces all of the problems described.  These bugs are reproducible when utilizing the 9600M GT as well.  I have also seen the bug occur on other machines running Mac OS X 10.6 or later.