Ok, I figured it out 
My first fix only addressed part of the issue 
There was another inefficient database query happening when searching for groups in the chat filter. Depending on which groups matched your search term, the query could take a very long time to complete – sometimes exceeding our request timeout.
Interestingly, this only affected “regular” users and not “admins”, hence why I wasn’t able to reproduce myself 
When searching for groups, results are returned alphabetically. Admins can see all groups, so their first 10 results for “L” were small groups starting with ‘a’ (like “ai-personas” and other non-public groups). Regular users have more limited visibility, so their results included the large trust level groups 
, which is what caused the slow query.
Regular user sees:
- trust_level_0: 62,506 users
- trust_level_1: 34,494 users
- trust_level_2: 4,727 users
- trust_level_3: 39 users
- trust_level_4: 13 users
- plus some smaller groups
Total: ~102,000 users to load 
Admin sees:
- a*****: 4 users
- a*****: 76 users
- a*****: 0 users
- a*****: 2 users
- ai-personas: 138 users
- etc.
Total: ~240 users to load 