It is a extract kit and I top off with tap water in the fermenter.
So you're boiling about 4-5 gallons worth? Or less? How long
does it take?
It shouldn't take an extraordinary amount of time, the largest size burners in many such stovetops can output around 2000-3000 Watt, although their temp is continuously regulated. IOW, the elements cycle off when they reach their internal temp limits, even when set at the max. When it drop below the threshold they cycle back on. Not much you can do about that. Just start heating the water while getting your other stuff together. And keep the lid on while it's heating up, it reduces heat loss, especially when you have a fan hood running. You could insulate the lid, covering it with a towel, as outlined below.
Once it boils, can you easily retain that boil or simmer?
If not, you could leave the lid on part ways, and put a large, couple times folded over towel on top of the lid, to reduce heat loss there. Just be careful, you still got hot glowing burners around, so don't leave it unattended!