How do I find out what the thread is waiting for here?

I am working on converting my project to using URP and looking at the profiler to improve performance. Each step, I have three calls of Gfx.ReadbackImage because I am getting three images in my project, but the first one always seems to take nearly 10x longer than the others despite the images being the same size (they're the same original image but using different shaders). I don't see anything different in the render thread, just a large gap. I don't see anything in any other part of the profiler at this point in time.
I may be misunderstanding something, so I am trying to learn how I can go about understanding what is happening here. Thank you.
Unity 2019.3.9f1

 

I'm pretty sure it's waiting for memory (your texture data) to be copied from GPU memory into mainboard/CPU memory.