Refrigerator water dispenser runs fast then slow
I have found when you have a “burst of water” followed by a low flow, the inlet valve in the filter housing is not fully opened. The large filter area pressurizes over  time maybe only a few seconds then when you open the valve you get a burst followed by a trickle.
A possible other cause would be media that is two fine and limiting the flow. Pre soak usually eliminates channeling but I would look at the filter seating / length as a possible cause.