My son just did a science experiment for school in which he measured the time it took for 1 liter (actually 4 cups) of water to be heated from 53F to 190F (whole 'nother story as to why he chose 190F instead of temps closer to boiling). Note that the pot did NOT have a lid - otherwise he couldn't track the temperature as easily, though he ran one test with a foil 'lid' to see the effect.

He did this in the garage (to block natural wind) with a fan blowing on low - measuring the wind speed with a Brunton wind meter to be about 3.5 mph - a VERY light breeze at most. It was about 55F at the time.

For the experiment he used an MSR Whisperlite running clean Coleman fuel, kept the valve at the same level, plenty of time after priming/startup, and did frequent pumping to make sure pressure was not an issue. He did two runs with the windscreen on, and two with the windscreen off. Then he did one more with the windscreen on AND a 'lid' made out of foil (so he could poke the digital thermometer through it).

Halfway through he had to refill the fuel tank, but did his best to put the valve at the same setting and checked to make sure the flame was similar in size (actually the valve didn't seem to change the flame output after some point in turning).

The results were as follows:

With Windscreen: 5:23 (min:sec), 6:16, 8:00
Without Windscreen: 14:49, 17:42, 22:04*,
Windscreen & Lid: 4:14

*He actually gave up after 22 minutes because he simply couldn't get the temp to go above 180 degrees.

These are real times, with only a light breeze, heating 1 quarts of water, on a mildly cold day. He actually started with the fan on high (5.5 mph), but the temperature wouldn't go above 170F without the windscreen, so he switched to the lower setting.

The 190F was selected because he ran a w/o windscreen first and he started to get frustrated after 16+ minutes, so he stopped the timing when it got to 190F and chose to use 190F for the rest of the runs.

So, if you think you don't need a windscreen, I hope you have NO wind and its quite warm outside. Otherwise, you're in trouble.

I'd strongly recommend a stove that can be used with a windscreen - or has one built in. Many of the stoves that sit on top of a fuel canister cannot use a windscreen.