Poor performance on nested images

[keflavich]

Describe the bug
CASA's imageconcat produces .image (and other) files that contain other .image files within, and these load extremely slowly.

If I try to change frames while the spectrum is loading, I have to wait until it's done before the image changes - and this can be 5-10 minutes on typical cubes (56 GB)

To Reproduce
Steps to reproduce the behavior:

1. Create an image cube comprised of many sub-cubes using imageconcat with mode='m' or mode='c'
2. Try to view a spectrum
3. Wait.

Expected behavior
Performance comparable to that of "normal" cubes, and/or, an option to stop loading the spectrum partway through.

Platform info (please complete the following information):

• OS: RH
• Browser: Chrome
• Version 4.0b1

Additional context
I expect this is somewhat, but probably not entirely, hardware- and filesystem-dependent.

I'm going to work around the issue by re-making my concat'd cubes with mode='p', but it would be best if we could use both types.