Investigate slowness of certain Projects::WikiController#pages requests

Summary

Projects::WikiController#pages has started to appear in #mech_symp_alerts with certain requests having a latency in the as long as 37s (at time of writing).

These requests can be seen in this Kibana query.

Using Kibana we can see:

• it appears to be a handful of wikis (one in particular) that have high latency
• if we change the Kibana filter to display all actions for Projects::WikiController it is always the #pages action that has high latency. This action lists all the wiki pages and paginates the array.
• when viewed sorted by time, we can see the latency is quite spiky and inconsistent

Improvements

We should:

• Assess the reasons for the latency
• Fix them!