Creating new pipeline for gitlab-org/gitlab is very slow (24 seconds)

1. Go to https://gitlab.com/gitlab-org/gitlab/-/pipelines/new
2. Click "Run pipeline" button
3. Wait for the pipeline to be created (about 25 seconds)

pipelines_265024118_perf_bar_1614804178814.json.zip

This seems like a ~performance issue for Projects::PipelinesController#create

Possible solutions

1. Gitlab::Ci::Pipeline::Chain::Create takes a long time. It may help to remove the pipeline variables unique validation since it doesn't work correctly and it executes a database query for each variable. See #322386 (comment 669028095) for more details. MR: !69595 (merged)

Monitoring dashboards

• percentiles for objects allocation: https://log.gprd.gitlab.net/goto/f35924e226d4d25ef5e54b5529b0cbda
• percentiles for requests duration: https://log.gprd.gitlab.net/goto/80f6610c2ea7f0e8ab312771651e5cc6
• percentiles for database queries: https://log.gprd.gitlab.net/goto/3a0a13f47669234e72bcc36345659c36