For me, fasting in Ramadan has reduced my eating frequency and I noticed that every year during this time my resting heart rate drops significantly. It kinda tells how much your heart works on your digestion.
It doesn't signify your heart slowing down because it doesn't have to digest food. A low heart rate suggests low metabolism (response of body to low amounts of carbohydrates available to it in order to be converted readily to ATP molecules and save them in case of emergencies)