There's no chance at all that you're using boiling hot water (373K) because that stuff is both dangerous and unreasonably expensive to make at volume.

My guess is that what you're thinking of is the water being say 320 to 340K, which is obviously far warmer than you'd heat a home, but isn't boiling water. Heat pumps will be more efficient at lower temperatures, so maybe it's keeping that tank of water at 320K while your gas boiler was 330 or even 340K -- but the gas boiler is more efficient for lower temperatures too, ask any manufacturer or installer, the way to get lower costs is to turn down the local thermostat, it's just that people are used to "instant" heat from a gas boiler and won't tolerate answers like "Run it for a whole day" whereas that is what your heat pump needs.

It is true that because of this "Run it all day" approach an uninsulated home is so expensive to heat that it's impractical, when I grew up there was ice inside my bedroom window when I woke on a winter's morning, it had been under 270K at night, and a gas boiler would heat it in an hour or so -- but that's not because it's literally impossible with heat pumps it's just far too expensive.