We have provided a registry protected fix, some users confirmed that this worked for them.
Could you please try this at your end as well and let us know your feedback?
Kindly make sure you are on the latest DC build(20.012.20043) and then follow the steps from the attached word file: “Steps.docx”.
An error occurred while saving the commentDave J commented
Any pdf, any size with comments or no comments takes a extremely long time to open the comment window. It's been like this for a few years. Deleted prefs and relaunched Acrobat DC. No improvement. Still the same. Currently running El Capitan, 10.11.6 on a Mac Pro 3,1.