Poor performance of WIkiDirectories Controller

Affected Controller: Projects::WikiDirectoriesController#show

Summary

The WikiDirectories controller exhibits poor performance, with p95 latency of 60s, and p50 of over 50s.

See: https://log.gitlab.net/app/kibana#/visualize/edit/AW2v4uVAQ7jyVXjiDMNA?_g=h@dd24b30&_a=h@2583f2b

What is the current bug behavior?

Median response times are very slow.

What is the expected correct behavior?

Requests should complete well within 30sec

Relevant logs and/or screenshots

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Output of checks

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Results of GitLab environment info

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:env:info)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)