PhotoshopServer is a Premiere component, and it's part of our Photoshop importer plugin. We've isolated the psd loading code into a separate process in order to reduce memory fragmentation. So it's quite normal to see it sitting idle, as it's only active when using psds, and it's only loaded when required.
OSX's activity monitor is a little misleading. It's meant to be there, and it's not being used. You will only see activity out of PhotoshopServer when importing PSD files into Premiere.
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