>
> I think it would be more useful to explain what the user can (and can 
> not) expect from those "specific optimizations".  Otherwise, it is not 
> possible to make an intelligent decision about the value of that option.
>

One of the design principles of that feature is that it should be as 
transparent as possible, and it is.  The point of adding a variable to 
control the feature was to avoid introducing an arbitrary constant at the 
C level, and also to allow users who really enjoy slowdowns to turn these 
optimizations off 😉  Its default value should be fine for everyone, there 
is no "intelligent decision" to make here.

>
> For example, describing that vertical-motion is still slow when point is 
> on a long line would be very helpful.
>

Vertical-motion is not slow anymore.