I think that on scrolling of 1000 lines, smooth scroll is not
necessary.  User wants smooth scrolling only for the first spin of
mouse wheel.

This patch introduces a new variable `pixel-dead-time' and
`pixel-last-scroll-time'.  When another scroll request was delivered
within `pixel-dead-time', very likely user does not want smooth
scrolling.

On such situation, `scroll-down' is called instead of
`pixel-scroll-pixel-down'.  On theory there should not be lag because
of smoothing.

I tested the revised pixel-scroll-mode for a week and confirmed that
works good.  When `pixel-dead-time' is zero, its behavior is the same
as before.  I think `pixel-dead-time' 0.1 works better.

I'm sending ChangeLog and a patch relative to the current master.

* ChangeLog

Add a new algorithm to avoid lag when scrolling is in rush

* lisp/pixel-scroll.el (pixel-scroll-up): Invoke 'scroll-up' when called within 'pixel-dead-time'.
(pixel-scroll-down): Invoke 'scroll-down' when called within 'pixel-dead-time'.
(pixel-dead-time): Interval that requires for the next smooth scrolling.
(pixel-last-scroll-time): Time when the last scrolling was made. (Bug#29737)