In the context i work, we did felt some slowness in September/October. Our batch has the size a slightly larger than yours, and we execute 5 batches in parallel. In good days, it takes 7 hours and a half to reach finalized state.

While we are not getting stuck in preprocessing in recent executions (during this month), we do get stuck in finalizing state. If the process stays X hours in this state, we delete the batch and call it a timed-out process. The same was done in preprocessing state if it stays too long there. Thats all we can do about that so far to make sure we dont get to a never ending process.

We did not had from them any info regarding endpoint updates.

When you said:

I'm pretty sure mailchimp deletes any outstanding batches 14 days after they were created

Do you have any documentation on that? I did not find anything related to that in mailchimp documentation.

And do you know technically what happens in preprocessing status? I am trying to decipher what is 'smaller operations' here.

Sorry to present more questions than answers :)