Investigate making search async (search functions run in parallel)

Django 5 made signals async:

https://docs.djangoproject.com/en/5.0/topics/signals/#receiver-functions

Search is independent and read only, so as far as making things parallel (which is always hard) this should be less hard.

Look into whether this works, how much faster it is, what we have to do to deployments etc to make things async etc