Test Case importer takes too long to load the custom fields mapping page

Description
For Jira instances with a high number of custom fields, the Xray Test Case importer may take too long to load all custom fields on the mapping page.

Pre-conditions
Jira instance with >7000 custom fields

How to reproduce

1. Open Xray Test Case Importer
2. Do next until the mapping page

Actual result/Attachments
Takes ~1 minute to open