@stumped has already directed you to threads talking about slowness to transfer books, this is the entry FAQ (written by the previous owner) that is mentioned in those threads:
http://cc_faq.multipie.co.uk/index.php?solution_id=1094.
Related to the subscription model, unfortunately is true, the new owner is migrating to that. If you purchased the app previously, you "might" continue using it with a nagging pop-up to subscribe, it seems that is what they ate doing in IOS now.
As I have older versions of Android, for now I'm not going to update the app, but I'm already looking for alternatives to CC because the older version probably won't work with Android 10 due to changes in Android permissions for apps. Chaley couldn't find a workaround to make the app continue working as now, so he stopped development and finally sold it. I think the main problem was transferring the books to a reading app, so if your use case is just consult in CC what to read on your Kindle, it might be possible you won't be affected by the changes in newer versions, and paying for the subscription or the nag screen while it works without subscription will work for you.