When multiple transmissions are in progress between a pair of systems, the files are multiplexed across the INMC link between the two systems to make full use of the bandwidth available. This shares the available bandwidth equally between the transmissions that are taking place, so that the more transmissions running concurrently to a particular destination, the longer each individual transmission takes.
Although initiators can be set to high priority, which expedites their transmissions over standard priority ones, these transmissions are competing for bandwidth with any other transmissions that are running.
Planning should include consideration of the number of transmissions to one destination that can effectively be run concurrently. While it is true that one transmission runs fastest by itself, operationally it is not necessarily most efficient to wait for the end of one transmission before starting the next. For example, if a report file transmission is in progress and will take four hours to complete, there is no reason why other small transmissions should not be run at the same time, rather than waiting until the report transmission completes. You can multiplex concurrent transmissions.