The exercise is to attempt a method in which you save computation of
or sha1, as these are one of the time consuming steps of createrepo.
The save would be in a 100k package repository: (100,000 - N) *
Time(sum_calc), where N equals the number of packages that *need* to
generate sums for. A parameterized list of package names passed into
createrepo would be sufficient to figure out what composes the N list.
An external process, such as a Manifest list, would then be used to
mitigate a set of packages through the entire build process. Apt uses
a md5sum cache, but having fine-tuned controlled of the process would
be more stable and directed. This is how much saving you'd get for #2.
Let me know when you've figured it out but as it stands I don't think
incrementally updating the metadata is very feasible.