You'll notice
Useful2 Signal1
Forking a conversation preloads its message index, so the first lookups skip re-reading the file.
What's wrong with this entry?
Anonymous. No account, no email.
What
After a fork writes the new transcript, the set of message ids for the new session is inserted straight into the transcript store's in-memory cache, so later lookups do not re-read the file just written. Insert is skipped if an entry is already there.
Evidence
primeSessionMessagesCache
Strings lifted out of the shipped bundle, so the claim above can be checked against them.