mouse-scroll-delay

mouse-scroll-delay is a variable defined in `mouse.el'.
Its value is

0.25

Documentation:
The pause between scroll steps caused by mouse drags, in seconds.
If you drag the mouse beyond the edge of a window, Emacs scrolls the
window to bring the text beyond that edge into view, with a delay of
this many seconds between scroll steps. Scrolling stops when you move
the mouse back into the window, or release the button.
This variable's value may be non-integral.
Setting this to zero causes Emacs to scroll as fast as it can.

You can customize this variable.