Use a cover
Evaporation is the largest single heat loss from a pool. A cover reduces heat-up time and run hours more than any other single measure, and it lowers the load on the heat pump.
Maintain filtration first
Pool heat pumps depend on adequate water flow from the pump and filter. A dirty filter or weak pump will present as poor heating performance.
Watch water chemistry
Aggressive water chemistry damages the titanium exchanger and pipework over time. Keep pH and chlorine within normal ranges.
Seasonal service
Before winter, have the coil cleaned, electrical connections checked, flow verified and the refrigerant circuit assessed so the unit starts the season at full capacity.