Heat pumps lose efficiency at lower air temperatures. Your BTU/hr rating is based upon what it can output at 70 degrees, but at 50 degrees it's about 1/3rd less and falls off a cliff as it gets cooler than that. Between the cold air and likely still very cold ground this isn't a surprising result.
Heat pumps are efficient and great for slowly heating and maintaining water temperature, but heating nearly freezing water quickly in nearly freezing weather is still a job for gas heaters.
As an aside: why are you trying to heat the pool right now? If you won't be swimming then it's going to take a lot of energy to warm it and warmer water will allow faster algae growth so it will use more chlorine than cold water.
Hello , This thread has been inactive for over 60 days. New postings here are unlikely to be seen or responded to by other members. For better visibility, consider Starting A New Thread.
Welcome to TFP! For a wealth of pool care information please check out our free Pool School. If you are in need of specific help then we encourage you to Register and ask us here in the forum!