When importing PDF Comments, it is mostly a breeze until the editor/author applies a 'Sticky Note'.

Moving through a document where 'Sticky Notes' have been added, the page, and surrounding pages, slows InDesign to a crawl and can take a few minutes to apply one change.

While my workaround is to run through all the 'Sticky Notes' in the PDF file first and apply those changes independently before deleting them, I can then import the PDF Comments in the usual way to speed up the remaining part of the process without being hindered.