1. A computer-implemented method comprising: accessing a cumulative feature store storing feature values determined from user actions performed before a time point; accessing an incremental feature store storing feature values determined from user actions performed after the time point; calculating, by a computer processor of an online system, a weighted combination of feature values from the incremental feature store and corresponding feature values from the cumulative feature store; updating the cumulative feature store by replacing the corresponding feature values in the cumulative feature store with the weighted combination of feature values; receiving an update to one or more of the feature values stored in the incremental feature store, the update comprising a new value for one or more of the feature values; and updating the incremental feature store based on the received update by replacing the corresponding feature values in the incremental feature store with feature values in the received update.