Requests to `/jira/rest/internal/2/user/mention/search` where the parameters do NOT include a query are very slow

Issue Summary

We had an outage on a Jira instance running JSD 3.16.11 today which appeared to be caused by requests to the `/jira/rest/internal/2/user/mention/search` endpoint.

Looking at request times for different queries, those that do NOT contain a query string appear to be significantly slower.

Requests with a query string:

Requests WITHOUT a query string:

Note the scale change - the longest with a query string was 13s, the shortest without was 28s, with longest over 60s

Examples:

1. With a query string, took 0.14s: /jira/rest/internal/2/user/mention/search?issueKey=KEY-5678&maxResults=10&query=il&_=1587494420052, (referrer is /jira/browse/KEY-5678)
2. Without a query string, took 62s: /jira/rest/internal/2/user/mention/search?issueKey=KEY-1234&maxResults=10&_=1587407226218 (referrer is /jira/browse/KEY-1234?filter=-1)

Steps to Reproduce

I am not sure what this endpoint is - I went into the referrer (for a lot of them this was filter=-1) and clicked around but was not able to hit that endpoint with anything I did.

Expected Results

Searches return promptly with no performance impact.

Actual Results

This actually caused an outage for us due to a spike in CPU which seems connected to GC activity.

CPU spike:

Unhealthy hosts:

Increased request latency:

New Relic showing that this endpoint is the slowest:

CPU activity by thread showing that GC is the main consumer of CPU. NB: we got the threads long after the request returned/timed out - it is likely still processing in the back end at this point:

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available