Importing large Rhino files will cause the Editor to stall during the import process for a long time. When testing the attached file the Editor stalled close to an hour with no results. The attached file opens in a few minutes in 4.19.2 (0.182).
This was tested and reproduced in 4.20 (4.20E4) and 4.21 (4.21E0)
Results: Importer tool stalls between 90% and 100%
Expected: The file is successfully imported
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-65247 in the post.