The Test Case Importer is very slow when the instance has a larger number of custom fields

The Test Case Importer is very slow when the instance has a larger number of custom fields.

Steps to reproduce:

1. Have a larger number of custom fields (of any type) in the instance. E.g. 1000 custom fields.
2. Import issues using TCI, either CSV or JSON format (no need to map any custom fields). E.g. 100 Tests mapping only the issue type, the Test type and the unstructured definition.

Expected result: The import takes a few minutes to finish.

Actual result: The import takes too long, e.g. around 15 minutes for 100 issues and 1000 custom fields. The time increases proportionally to the number of issues being imported, the more issues, the longer the import takes.