1st factor would be the water's volume
2nd would be the Temperature
3rd would be the rapid increase or stability of temperature
=If the water's volume is low then the boiling process would be faster...
=If the temperature of the heater is too low then the boiling process would be even slower... and much faster if it's too hot....
=Tap Water's Boiling Temperature is 100 %C ,Increase in temperature higher than 100 %C makes water boil a lot faster