/api/:version/projects/:id/issues/:issue_iid is slow

Nearly 100% of the observed slow endpoints are from gitlab-org/quality/testcases. The full logs are available at https://log.gprd.gitlab.net/goto/df12deee4dbf0ad290aacb891563e4c5.

This issue can be manually tested with this command:

http -v https://gitlab.com/api/v4/projects/gitlab-org%2Fquality%2Ftestcases/issues/1085 Authorization:"Bearer $MY_GITLAB_TOKEN"

It constantly takes 10-15 seconds waiting for the result.

An example can be accessed at gitlab-org/quality/testcases#1085 (closed). The page constantly returns 500 errors. The XHR endpoints are different though.

One note is that the user agents of thoe endpoints are mostly from a gem. I guess they come from our internal workflow.