is this your pump ? Quiet One 2200 Water Pump: Aquarium Supplies - Aqua Buys
If so, it's rated at only 581 gph
before head loss.
I still think a Mag9 does you well. And they are great pumps.
Vertical and horizontal length all act against the pump pushing the water. vertical because the pump is pushing water up against gravity, horizontal because of friction. Diameter matters because much like the drain there is a physical maximum that can be delivered based on the diameter. Elbows/ unions also count for head loss.
Think about blowing water through a straw or hose. Is it easier if the hose is short or longer? If it's str8 across or str8 up? If there are a bunch of turns or completely str8? If the hose is bigger or smaller?
Same basic concept.