Figure out cause of timeouts for full list-images, and adjust

[squaremo]

We want to make more use of the all resources list-images API, but in our dev environment for instance, it tends to time out.

Possible remedies:

• if there's too much data, introduce a summary mode or the like so that we send only (say) the current image, the latest (filtered) image, and the distance between them.
• if there's too much calculation, um ... figure out how if we can do less calculation.

[foot]

Related #913

[squaremo]

Ops! Closing as duplicate of #913.