Decant? Do you mean sump? Or skimmer? Faster water through the sump won't really impact things and not enough contact time or too much contact time can impact the skimmers performance but when the skimmer is properly tuned, the amount of scum is limited by the amount of dissolved organics in the water.
When the skimmer is oversized relative to the fish load, the skimmer will not perform consistently because there won't be enough dissolved organics (Proteins) in the water for the skimmer to be able to produce enough foam to keep the neck consistently filled with foam. The proteins allow the bubbles to bind together in chains that create the foam. When there isn't enough proteins, the skimmer will just make bubbles and not foam. So too big of a skimmer relative to the load will produce skimmate (scum) for a day or two and then slow down or stop producing until the dissolved organics (proteins) from the fish waste build back up and allow the skimmer to produce enough good foam to keep the neck filled with foam and push the organics out into the cup. Then once the organics are reduced which will happen fast when the skimmer is way oversized for the load, the skimmer will just stop producing skimmate and the foam will drop a bit in the neck. During this time, the dissolved organics in the water will increase slowly over the course of days or weeks until the skimmer can generate a good foam head again and then the cycle repeats resulting in nutrient levels in the system that go up and down and up and down. It can take anywhere from a few days to a week or more for the skimmer to produce skimmate again depending on the load but your load is VERY low for this sized skimmer. As such, the only way you are going to see any kind of consistency is to run the skimmer VERY wet which means raising the level inside the skimmer. Increasing the pump speed by 2 watts or so will help skim wetter without adjusting the sump level or having to close the wedge pipe. You will remove more water by doing that but if filling your cup is important to you, then that is the only way to do it with such a light fish load. Speeding the flow through the sump won't fix your concern and speeding the water though the skimmer won't either. That skimmer is designed for much larger loads and it's starving.
