Birm has a recommended flow rate. The company that is the primary distributor is the most conservative company when it comes to media flow ratings. We use their data to make our system designs and specification sheets, but... most people greatly exceed the manufacturers numbers. I am showing that a 2 cubic foot birm system will remove iron at a flow rate up to 3.9 GPM. As you can see, this is a little tough to design equipment around, so regionally, people use their own numbers to determine the flow rate based on many factors. Mostly through experimentation and trial and error or pilot testing.
Here is link to Birm, it has a good amount of information and you can see how we determine our flow parameters.
You can use a softener to remove iron, anything above 2 PPM is rarely recommended, and it is a highly inefficient, but fairly inexpensive way of removing iron. I know many companies do it and sell on it, in order to reduce the up front costs. This is short sited, and is not good for our industry which has been under attack for 30+ years because we have been promoting inefficient ways of treating water.