Suggest you turn the unit off. Remove the drain plug on the unit. The water drained should come out clean. If there is a lot of scale coming out with the water you may have to buy a cleaning wand that hooks to a garden hose and you insert it into the drain opening and wash out the scale and calcium deposits from the tank.
If there is a lot of scale and deposits in the unit, that might be the reason for its lengthy heating time. My 10 gallon unit will heat water in about 10 to 15 minutes on gas. Takes much longer on electricity. As you can see from the above posts, the heating time does vary a lot.