Subsets taking a long time to finalize in Cubeviz on main

[pllim]

This was #3164 by @rosteen that got accidentally deleted.

See the video below - in Cubeviz (I confirmed that Imviz is behaving normally), subsets are taking up to 5-6 seconds to finalize when drawn, which is rather annoying. I haven't had a chance to track down why this might be happening.

Screen.Recording.2024-08-22.at.12.02.20.PM.mov

I found an explanation but don't have a solution yet:

#2939 introduced subset_selected_changed_callback=self._update_extract to both the aperture and background selection menus in the spectral extraction plugin. Unfortunately, the SubsetSelect component in template_mixins.py calls subset_selected_changed_callback whenever it gets a SubsetUpdatedMessage. Whenever a subset is created, a SubsetUpdatedMessage is sent for EVERY item in the data collection, meaning the the entire spectral extraction process (via _update_extract) is getting called twice for every item in the data collection any time a subset is created. Note that this includes getting called twice for every extracted subset spectrum (including both previous subsets and the one that was just created), explaining why creating a subset gets slower and slower as you make more of them.

[pllim]

@rosteen noted that #3175 only marginally improved the performance.