Solving performance for cross-app comments

[theDanielJLewis]

(This is one of several posts that I'm starting to focus on solving specific issues we may have preventing full support for cross-app comments.)

One of the problems I've seen with ActivityPub for cross-app comments is slow performance, especially if comments can be left from separate ActivityPub instances.

Caching is an obvious solution, but how?

I think there could be two approaches:

1. The podcaster uses a service (perhaps provided by their podcast-hosting provider) that caches the full activity stream into an authoritative source. Other apps can use the cached source for the initial load, and then do their own checking of the full activity stream to find any additional comments that might not have been cached, yet. But even that might be unnecessary if the cache source can be triggered to refresh by any podcast app sending a cross-app comment.
2. The separate podcast apps maintain their own cache, but this probably wouldn't work for serverless apps.