Since it's append-only, can't you "cache" or snapshot the calculation in pieces? e.g., "Today @ 00:00 all events on this account summed up to 100." Then you jus

Since it's append-only, can't you "cache" or snapshot the calculation in pieces? e.g., "Today @ 00:00 all events on this account summed up to 100." Then you just replay today's events. You don't have to recalculate anything unless you're changing your transformations. Make your checkpoints whatever interval is necessary to make on-demand event replay acceptably performant.