Extremely slow after ~50% of repo are analyzed

[kkmuffme]

I have a repo with ~70000 commits.
--analyze is rather quick (minutes) until it reaches about 35000 commits, and from that point on it takes ages (= days)

a) the commits I want to change are all in the same branch or sub-branches of it and less than 2 days old.

It would be great if there were an option to limit analyze to e.g. a specific commit and commits after it?

b) any idea what could cause this slowness?