Yes you have to run the water on full pressure for the duration of emptying the tank.
In the summer when I do my water changes, I actually have a DIY cover which fits over the outlet of my python. What this allows me to do, is instead of wasting the water, I divert the outflow to another hose which then goes into a 110g garbage pale outside my house. Once a week, I stick a water pump with hose attached to the end into the pale and use the water for watering my garden / flowers etc. Saves me from ever having to run the outside faucet.
I made the DIY outflow pipe using a 1 1/2" tube which I got my local hardware store, a small "worm gear" clamp (see here
). This fits over the end of the python and almost goes up to where the python hose attaches.
Now that I have just started a saltwater tank, I will use some of the water for watering for my garden, the rest I will pump through my RO/DI unit and if it comes out clean enough (havent actually tried it yet), will be using it for my auto top off system for my saltwater tank. If the water does come out clean enough then I will not be wasting anymore water even during the harsh Canadian winters. If it is not suitable to use in my RO/DI unit then I will have to send i down the drain as I cannot run the pale outside in winter and have no room for it in my house.
Just a thought as to what you might do with the water.