Taylor Blau [off-list ref] writes:

This improves many cases where using bitmaps was significantly *slower*
than regular, non-bitmap traversal. In some instances, using bitmaps is
still slower than the non-bitmap case, but it is a substantial
improvement over the classic bitmap walk.
...
In a large repository on GitHub.com, the timings to compute the objects
unique to "master", as in:

    $ git rev-list --count --objects master --not --exclude=master --branches

improve as follows:

Curious---when would it be significantly faster to use bitmaps?
"Most of the time"?  "In not-too-large repository?"