Agreed. I notice better skimming in my pool when the flow is throttled down a bit. AS far as your solar panels, the faster you water goes through them, the better. A good way to know what's going on is to put your hand on the panels. If its a sunny day and the panels feel hot, your system is inefficient and not transferring all of the heat energy to the water running through. If the panel feels cool, than the system is efficient and transferring more heat to the water.

The problem is most people go by the feel of the return water. When the system is running slow and inefficient, the water at the return may feel warmer than when it is running fast and efficient. But running it slow is like pouring a pot of scalding hot water in a bathtub full of cold water. It won't do much good at all, bath water will still be too cold to get into.