The get_auth_chain_difference transaction can often take hundreds of seconds of database time (during which all other traffic to the room is queued). This is a large component of database load from state resolution.

Caching the results of this transaction wouldn't be useful as each result is typically used only once (to populate the cache of state-resolution results), but could we cache some of the inputs which could get reused across multiple transactions (for example, the list of auth event ids for each event)?