Mark Tucker

There is going to be some additional time to load files like fbx or obj, translate those files into equivalent SOP form then USD form, running the LOP nodes... But this time is going to be pretty inconsequential compared to the time spent actually rendering.

So I'd advise doing the thing that makes the user workflow as smooth as it can be. Having to pre-convert source data into USD files, or pre-cache lights into a USD file are annoying extra steps for the user (or for you while you work on this HDA) which will cost way more time than doing the geo translation and light creation "just in time".

This balance may switch if the geometry is incredibly complicated. Of course the more complicated the data the longer the render takes, so it's still a small percentage of the time spent. But this time may become noticeable to the user. So don't actually trust anything I say. Measure how long these various steps take with your actual data.

This has nothing to do with the switch, AFAICT. The difference is that on the "doesn't work" side, you have set the Output File to be blank. So the root layer of the stage (the one that adds the BASE_LAY as a sublayer) isn't being written out. So the SETDEC_LAY on the "doesn't work" side is simply not sublayering in BASE_LAY. The simplest fix here is to change the sublayer2 LOP to turn off "Edit Root Layer". This will add the BASE_LAY layer as a sublayer of the SETDEC_LAY layer you are authoring/altering with the cube/sphere/pig LOPs.

There is no reason this shouldn't "just work". The Switch LOP input index parameter can definitely be animated. The time dependence may disappear downstream if you have a Cache LOP between the Switch and the ROP, but then the Cache LOP would have already cooked the animated switch value. Keep in mind that USD scene graph hierarchy cannot change over time, so if one switch input creates /cube and the other switch input creates /sphere, and your input index is "$F%2", saving out the first two frames will result in a USD file with both /cube and /sphere in it. But the ROP has the "track prim existence to author visibility" which will mark the sphere invisible on frame 1, and the cube invisible on frame 2.