Discovered a bug where strong filters were causing a big slowdown effect on mouse hovers over output nodes. This is because CoreImage turns out to be lazy in it’s computation, and the NSImageView used to show the output image was asking the CIImage each time it changed panes (which I was using to highlight it) for a representation that needed processing. Remedied by adding my own subclass of NSImageView that does it’s own highlighting using a subview.