Photoshop: Why are scripts, actions, and batches modal?

I completely understand your frustration. With Lightroom it's even worse. I have to run Lightroom on 4 cores max while I have 6 cores in my machine. At least if I work a long time in one stint in Lightroom.

That said, it's not entirely trivial to scale a process across multiple cores. Especially considering the multi threading which is happening on modern PC's. So a 4 core machine virtually has 8 cores...

I'm coming from 3D and there all renderers are multi threaded. Especially ray-tracing scales like a charm. But either you launch the rendering in a shell, or the main application like Maya launches an external exe to render.

The downside of this is that you'd have to manually limit the external exe to only use that many cores that you can still work on that machine. But something like a gaussian blur filter will probably never run multi threaded. Anyone who knows better, please correct me if I'm wrong.

I think you have hit one area where something could be done. Batch processing could happen in the background. But my gut tells me it's still the photoshop.exe that is used to run the batch. And if that exe is busy, it can't handle additional tasks, respectively run a second instance of itself. They would have to run a separate exe so that the main exe is idle and detach the UI. Again, not entirely trivial afaik.

Off course you could use another tool like Faststone Image Viewer to batch process the scaling. But I know that's not what you're after.

I've been following your thread which is very interesting. I'm running PS on an old 6 core I7 with 12g of ram, SSD. Recently I was looking at newer systems as I noticed a slowness in PS at certain times. I came across an add that said PS runs better on 2 cores than it does on 4, 6 or more. It also stated a way of assigning which cores where to be used.  I tried limiting PS to just 2 cores out of 6 and found that it did run better. FWIW!