Problem

When loading any board's "Configure" page, it can take an extremely long time when JIRA has a lot of custom fields. This looks to be regardless of the board configuration

Steps to Reproduce

• Create an Agile Board
• Add 4000+ fields to JIRA
  • This can be done via the Data Generator: https://marketplace.atlassian.com/plugins/com.atlassian.jira.plugins.jira-data-generator/server/overview
• Load the board's "Configuration" page

Expected Results

• Configuration page loads within a reasonable time

Actual Results

• Any board's configuration page takes an extremely long time, 20s+

A Jprofiler snapshot, board_configure.jps, is attached and we noticed that FieldSelectorResult.accept take most of the time to process.