Slow response from PipelinesController#show.json

ZD: https://gitlab.zendesk.com/agent/tickets/113459, https://gitlab.zendesk.com/agent/tickets/112439

Breakdown

16% getting Ci::Pipeline#legacy_stages (profiling overhead is part of this):

11% preloading Ci::Pipeline relations:

10% generating paths:

7% checking BuildActionEntity#playable?:

3% due to repeated Gitaly FindCommit calls:

3% rendering JSON (can we switch to Oj instead of ActiveSupport?):

Edited Feb 28, 2019 by Stan Hu