Bug: People page is unusable at scale

[ibrand]

Describe the bug
We see that once there are thousands of texters in an org, the people menu will slow and eventually give the green spinner and never resolve. This renders that page totally unusable. Unfortunately, I'm not sure at what point the load is too high.

To Reproduce
Steps to reproduce the behavior:

1. Have a large number of texters (maybe over 1k? Maybe around 4k?)
2. Go to the people page
3. See error

Expected behavior
The people page should load quickly, and allow the filters to sort the texters

Additional context
This was first witnessed on MoveOn's Spoke instance which has around 4k texters. That's why I'm unsure of what point it fails. It would be a great first step to figure out how many texters it takes to hit that failure state.

[ibrand]

This is included in #1538