Of course, because it is. It's going to do a tiled render, likely at powers of 2, and all the heavy many-layer PSD stuff is completely asynchronous from the actual panning & zooming.
Or are you so naive you think that anything of significance is happening on or blocking the UI thread such that merely showing "smoothness" in any way reflects anything about the device's performance at actually doing the photoshop work?
That's like wondering why the source + instrumented dev environment uses so much more resources than its compiled, optimized binary.
What a weird confusion to see on HN.