If you cook in normal stove 1 liter of water, it may take around 10–15 minutesto boil depending upon the stove that you are using.
If you use Water Heater to cook 1 liter of water it takes around 4–5 Minutesto boil a liter of water. (Of course it also differs from water heater to water heater)
If you want it super fast, usually microwave oven takes around 3–4 minutes to boil water ( I used the level 5 power and it is a normal microwave oven).
Vs an electric 2400W (so they say) heater
couldnt boil water without a lid.