Okay, some real world numbers. First, the caveats. The pump is old, like 15+ years old. The impeller shaft is really grooved (I looked at it today for the first time in forever after seeing how bad the numbers are). I timed each combination filling a 5 gallon bucket to overflowing.
Python (40') gravity feed - 1.28 GPM
Python (40') with pump - 3.22 GPM
Garden hose (50') with pump - 5.17 GPM
Pump with short garden hose (3') - 11.54 GPM
As you can see the Python hose with the smaller diameter really restricted flow. But even wide open the pump is only pushing about 700 GPH vs. the listed 1800. Like I said, it's old and the impeller is badly grooved. I've had diminishing returns for years but didn't realize how bad it has become.
The big takeaway would be to run the shortest hose, with the largest diameter, and to try to keep it as straight as possible (avoid hard 90's and things like connectors that restrict flow). Head pressure is a bitch.
This might has pushed to to make a custom water change hose with a larger diameter. I usually run the Python to vacuum debris while pumping out water through a garden hose at the same time.