tree expand speed in explorer

[youhou 0]

ok right, i've searched the forum but found no post about this one :

 

how do you change the speed of the expand in the explorer tree view.

 

i.e. i drag a file from a directory to another, but since the target directory is under a collapsed one i have to let my cursor over it like 1 sec until it expands. and if i have more than 2 depth levels it takes ages to reach the target directory.

 

can anyone give me a clue on this one.