Yes water hardness varies widely from one well to another. My record hardness is 136 gpg.
Bath tubs usually have the highest flow rate of any fixture in a house. And all other water being used at the time a tub is beiing filled is added to that flow and the softener has to treat all of it. Front loading washing machines are connected to the same hoses and plumbing in the wall that other washing machines that use more water are connected to. So it's the same gpm flow rate, regardless of the number of gallons used. Front loaders use the same gpm but for a shorter length of time.
"Soft" water in relation to residential softeners is stated in gpg (grains per gallons) and the WQA (Water Quality Association) says a softener is working as long as there is no more than 1 gpg of hardness in the softened water. I say it should be 0 gpg and I size for that.
The slippery feeling is easy to get used to when you realize your skin and clothes and appliances and fixtures are benefiting greatly from softened water. So is your water heater and shower heads and valves etc. but, if you want to, Clack plumbing connectors allow an easy simple way to add some hard water back into the softened water if you want to do that. Although I don't suggest it. She will be cutting way back on detergents, soap and shampoo and conditioners and her skin will be very soft, smooth and moisturized without those products. Her hair be silky soft'n shinny and the clothes will be soft and much cleaner and brighter too without adding laundry softeners or softener dryer sheets. And everything she cleans with the soft water will clean up easier and faster and stay clean longer. And once she gets used to it (about 3 weeks), she will really hate going somewhere and having to use hard water. And she will be your official soft water tester, if the softener starts allowing hard water through it, she WILL be telling you all but immediately.