I located the bug, in the recent introduction of `mouse-wheel-progressive-decelerator' into aquamacs.el.
There are two issues:
1) The requested scroll amount is divided by the `-progressive-decelerator' value, even when progressive scrolling is turned off.
2) Integer division results in zero scroll size requested,
a) always, when progressive scrolling is off; and
b) for the first few scrolls with progressive scrolling on.
The combination of (1) and (2a) is the reason that turning off progressive scrolling makes scrolling unresponsive.
I have submitted a patch and pull request that:
A) changes `mouse-wheel-progressive-decelerator' to a float value so division gives floating-point (untruncated!) result;
B) applies `ceiling' to the (float) result from division, because the `scroll-up' and `scroll-down' functions that are subsequently called will (apparently) only scroll a single line if given a float argument, regardless of value. And zero is never a reasonable amount to scroll, so we don't want to round down.
C) only divides scroll amount by `-progressive-decelerator' value when progressive scrolling is on.