One indication of a failing hard disk is a very slow startup . Before you do ANYTHING please backup the computer using Time Machine or create a bootable clone if you run Catalina or earlier. Then test your backup to ensure it works. Once you have the backup completed and verified that it works please do the following:
Please navigate to the Mac App Store or navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.
When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again.
For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211