bug#12463: 24.2; pos-visible-in-window-p gets slower over time

[Eli Zaretskii]

> I'm saying: is the variable usually set once and left alone, or is it
> usually let-bound around the call to the function that uses
> the variable?
> 
> In the case of dynamic-library-alist it seems it's never let-bound, so
> it's probably better to access it directly as a global var, rather than
> pass it as a parameter.

It is never let-bound.  It is modified by adding cons cells to it, and
used by accessing those cells.