It is not really possible to have the same batch settings for processing the queue with a cron job and through the browser. At the start of each batch phplist looks back to see how many emails have been sent in the past batch_period seconds. This ensures that the batch limit is not exceeded but also has the effect of reducing the sending rate.
You need to set the batch period to be a small number. Not sure whether 0s will actually work but, say, 5s will. You should also disable processing the queue through the browser because this change would lead to exceeding your batch limit.