> Scrolling through emacs-25's xdisp.c, I get the following timings:
 >    emacs-25 branch: 45.529s
 >    master branch:   25.529s
 >
 > This is a speedup of around 78%.

Thank you.  Using the attached test2.el on frame.c, I get the following
results for emacs -Q (‘foofoo’ scrolls up profiling ‘foo-1’, ‘foobar’
scrolls down profiling ‘bar-1’).

emacs-25:

                Call Count  Elapsed Time  Average Time
foo-1          169         109.15699999  0.6458994082
bar-1          169         113.032       0.6688284023

master:

foo-1          169          45.28100000  0.2679349112
bar-1          169          53.406       0.3160118343

These look even better than yours (I have no idea which formula you used
to derive a speedup of "78%" though).

martin