<str:project_slug>/ is slow for some projects

[madwort]

Spotted on bubble-up: the page for <str:project_slug>/ is slow, but seemingly only with a few specific orgs - worst offender is /investigating-events-following-sars-cov-2-infection/ , with opensafely-internal runner-up.

See also https://ui.honeycomb.io/bennett-institute-for-applied-data-science/environments/production/datasets/job-server/result/w6YcsXamFaW?bubbleup_result=t42SFETUwhr&tab=bubbleup

slow URL: https://jobs.opensafely.org/investigating-events-following-sars-cov-2-infection/

[ghickman]

The optimisation in #3400 doesn't seem to have made a noticeable difference.

@madwort has added some spans in #3401 to see if we can pick apart different parts of the view.

[madwort]

the repos span is the bulk of the request time, e.g. https://ui.honeycomb.io/bennett-institute-for-applied-data-science/environments/production/datasets/job-server/result/C1gNycu3F7t/trace/CUcWLoT8mts?fields[]=c_name&fields[]=c_service.name&span=ec7c10c24d66c474

[mikerkelly]

#4634 noted this issue and suggest a solution.