When you drag a segment, you normally get what Avid calls "Four Image Display," or "Segment Drag Quads." Sometimes these can slow you down when you have a long sequence or many effects, since it has to draw those images; and many times you could care less about seeing images anyway (like when playing with audio segments). So, how do you turn that function off?

 
In v7.x and later there is a Timeline Settings dialogue box in your settings menu. One of the choices is "Show Segment Drag Quads." Simply de-select this option.

 
But then what do you do when you want to turn it back on? There are two fast ways:
 
1) With your Timeline window active, type "COMMAND + =" and the settings dialogue box will pop up, allowing you to quickly check the box (by the way this will work for Composer settings, Bin settings, and Digitize settings as well; just make the appropriate windows active and type "COMMAND + =" – which is also a selection from the Special Menu.

  
2) This one is a little tricky, but is fast once you get it to work (and it works on earlier versions of Media Composer that don't have any other way to turn the images off). Here's the steps:
A - Select a segment with either flavor of segment mode
B - With the segment selected, continue to hold down the mouse button
C - Hold down the Shift key
D - Move the segment and you should hear a beep from the Mac. The images are now turned back on (presuming you already turned them off in your settings). Drop the segment and try the above steps again, and the images are turned off. So, I usually leave them off in my settings and turn them on only when I really want them. It makes moving segments around super smooth and fast.