No that's not the case.
We have a water supply 80 metres away for tonights work, decent flow. We will be able to start within 5 minutes because the water in is faster than the water out.
Even on a standard supply you should get about 15 - 20 LPM so you will slowly lose. Get half a tank full and then start pressure washing. Get your set up done and you should be nearly good to go.
It is your set up that gives you the speed. I don't know what yours is. Once I have a water supply I can be starting within 2 minutes of getting there.
Back doors open (no jokes), pull fill hose to tap/supply - turn on water.
Back to hose reel - extend high pressure hose, connect lance etc.
Is the supply going to keep up - yes - turn machine on!
No - give it 5 and give the client some 'blarney' about something and then make a start.
Give yourself a half tank start with your set up.
If you have a 21 LPM machine that will be more than adequate and is much better than I started with. It is the things that connect to the job that get you away from there quicker, not just the machine.
Rob