Issue Summary

The changes made in BSERV-20271 causes performance regression in pull request participant search.
The versions impacted are limited to: 8.19.26, 9.4.14, 9.4.15, 10.1.2, 10.1.3

Steps to Reproduce

1. Navigate the the pull request list page on a repository with lots of pull requests (closed or open it doesn't matter)
2. Type into the user selector a string for a username that doesn't exist; a long random string works

Expected Results

The request submitted to Bitbucket should complete in ~1s

Actual Results

The request takes many seconds. The profiling logs can also be inspected for slow calls to the participants endpoint:

2026-01-08 05:39:08,581 | http-nio-7990-exec-43 | *1P4JHU3x338x2172068x6 | someuser | 9otk65
[24979.8ms] - "GET /rest/api/latest/projects/EXAMPLE/repos/example/participants HTTP/1.1"
 [1.7ms] - ApplicationUser com.atlassian.bitbucket.user.UserService.getUserById(int)
 [24836.0ms] - Page com.atlassian.bitbucket.pull.PullRequestService.searchUsers(PullRequestParticipantSearchRequest,PageRequest)
  [24834.5ms] [count: 3, avg: 8278.2ms] - Page com.atlassian.stash.internal.pull.PullRequestParticipantDao.searchUsers(PullRequestParticipantSearchCriteria,PageRequest)
 [135.1ms] - AvatarEnricher.enrich
  [20.2ms] [count: 18, avg: 1.1ms] - void com.atlassian.stash.internal.avatar.AvatarUrlDecorator.decorate(Builder,ApplicationUser)

Workaround

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