Thanks for the reply. In looking over my CAD drawing, it looks like they use 4" pipes for both the spa and pool main drains (1.5" for vacuum and skimmer), and the returns are 3" to the pool, 2" to the spa return (drain), and 3" to the spa jets. My typical setup is 50/50 between the pool main drain and the skimmer, and 50/50 between the spa return and pool returns.

But the essential question I had you answered -- half-speed pump = double the time it takes to turnover the water. Given all that you said and all that I see in the CAD drawing, I'll assume that the water is being turned over at least once a day when I'm running it at 1800rpms. Thanks again!