Troubleshoot for OneDrive macOS sync / slow processing
Solution
macOS version prior to 10.14
If the macOS version is less than 10.14, it is not compatible with on-demand sync and will try to download everything to the user's computer, which in many cases is a lot of data and may even fill up the user's hard drive. This will cause a lot of slowness as the initial sync happens. macOS versions before 10.14 are not supported by Apple and should be upgraded to 10.14 or above.
OneDrive Client prior to 21.002
OneDrive Client may download all files during initial sync to a cache, prior to registering the Files on Demand setting. Once it registers the On-Demand setting it will remove the extra cached files. In many cases this will just make the initial sync slow, but in some cases the cache space or sometimes even the hard drives fill up and the sync will be unable to finish. In this case the preferences need to be changed to only sync specific folders or the OneDrive client needs to be updated to 21.002 or above.
Environment
MAC Operating System versions prior to 10.14 and 21.002