Cold water from the shower

Hi, my shower seems to take too long to get warm water from the hot water faucet. For example, my wife takes a shower, then I take a shower say 5 minutes after. The hot water is already cold and it takes a good 1 gallon to get warm water again from the warm faucet alone.

Is there an issue here or the water just cools off very quickly? I hate to use a gallon of water before even showering.