Is it possible to append the files outside of Ensemble and then use batch record map? The current version of Ensemble handles batch record maps much more efficiently than it can handle individual record maps. If you are on an older version, let me know and I'll find out if the efficiency improvements are in it.
I think that increasing pool size will give you some improvement, but batch record map can avoid a lot of overhead handling each individual record.
Part of the efficiency improvement depends on what you're doing with the record. If you're just loading data, it's very efficient. If you're performing a transformation on the incoming data then that could still take a significant amount of time.