Low speed of a two speed pump has half the flow rate of full speed. So turnover takes twice as long.

Depending your plumbing, the full speed flow rate for a 1.5 HP full rated pump will be at least 66 GPM so low speed would be at least 33 GPM.

BTW, I have a 1/2 HP two speed pump and the turnover for my 21k gallon pool on low speed is less than 11 hours.

Bear in mind that running on low, if you have an erosion style tablet fed pool will likely have a detrimental effect in many cases. The chlorine normally fed when the water slows down is also reduced and often needs augmenting. While this is not a big deal in most cases, it can get in the way of how you were doing it before. In other words, the motor change is not the only change. Please keep this in mind.

Scott

Never cared for them myself.....JMO