DB timeouts loading Security Dashboard

Performance issues are causing DB timeouts for the Security Dashboard on gitlab-org.

This can be reproduced fairly consistently by going to https://gitlab.com/gitlab-org/gitlab-ce/security/dashboard and watching for errors to appear. Refreshing the page typically causes requests to succeed.

The endpoints for https://gitlab.com/groups/gitlab-org/-/security/vulnerabilities/history and https://gitlab.com/groups/gitlab-org/-/security/vulnerabilities/summary are causing ActiveRecord timeouts.

Timeout logs in Sentry:

https://sentry.gitlab.net/gitlab/gitlabcom/issues/881389/?query=url:https://gitlab.com/groups/gitlab-org/-/security/vulnerabilities/history

https://sentry.gitlab.net/gitlab/gitlabcom/issues/881389/?query=url:https://gitlab.com/groups/gitlab-org/-/security/vulnerabilities/summary