I think it mostly is by not counting as much. In typical Objective-C code, you will find that only the UI is actual Objective-C. Also, many fields of Cocoa UI classes are 'plain old data' such as 'int', 'BOOL' or enum's. That keeps the number of objects down and decreases the amount of bookkeeping.
The GUI library also is smart enough to not allocate more objects than needed. For example, in a table, only the cells actually on the screen really exist, and all controls in a single window share a NSTextView called 'the field editor' that is used for editing text (http://developer.apple.com/library/mac/documentation/Cocoa/C...)
Finally, I do not think it is that fast. It is just modern hardware that is fast.
The GUI library also is smart enough to not allocate more objects than needed. For example, in a table, only the cells actually on the screen really exist, and all controls in a single window share a NSTextView called 'the field editor' that is used for editing text (http://developer.apple.com/library/mac/documentation/Cocoa/C...)
Finally, I do not think it is that fast. It is just modern hardware that is fast.