I'm not so sure that is not a big drop.
the pump output for that filter is 900 gallons per hour.
now, we don't know under what conditions this is - it is probably just the pump, not hooked up to a filter, on a tank etc.
so, assuming 900gph under ideal conditions, setting it up on a tank with clean media you still probably have something like 700 gph new..
so a drop like that could mean 50% which is understandable after a year. but I wouldn't say the drop is minimal.
I found that with the eheim wetdry that I used to use, the flow rate would drop off noticably after a few months.
and not surprisingly, the media was quite dirty when I serviced it.