Time Machine extremely slow to free up space after Catalina upgrade

Unfortunately, this is a common issue with Catalina. Apple made a number of significant changes to Time Machine in Catalina, but neglected to document them for users, so there is not much to go on here.

I experienced the same kind of issues when I first "upgraded" to Catalina and found that the only thing that really helped things was to erase the Time Capsule disk and start all over again with a new backup.

Even then, the "Preparing" message appeared for 3-4 hours before the actual backup began and the backup itself took much longer than a complete backup using previous operating systems.

Once the iniitial backup completed, subsequent "incremental" backups seem to be running OK, but still slower than in previous operating systems.

If you decide to erase the Time Capsule disk and try starting over again, and need a few more tips on that, post back for more details.

It also might be a good idea to consider a different type of backup in addition to or instead of Time Machine.

FWIW. We seen a number of posts with folks having issues with Time Machine (TM) backups to their Time Capsules (TC) since upgrading to macOS Catalina. It appears that original TM backup set's filename gets changed on the TC from .sparsebundle to .backupbundle, but something else may be going on during the "Preparing Backup" process that is causing the increased time required to complete successfully other than just the name change.

As such, we are seeing that TM is becoming less reliable with every new release of macOS and some of us have been recommending an alternate incremental backup solutions, like Carbon Copy Cloner.

Did you reformat the External drive APFS?

Not the old Mac OS Extended Journaled.

Apparently Catalina now requires APFS although ... as others have commented.. info is scarce.

It won't be an issue to network drive as the local computer does not control the format of network storage and only uses a protocol to send the files to the device which handles the data transfer to the disk.

I am having EXTREME performance problems with TimeMachine on Catalina. I am backing up to an external TB2 mechanical drive. I erased the drive and reformatted with a fresh volume. I am backing up about 3TB of data from scratch. Watching the progress bar on time machine the bar looks essential frozen. Over the course of an hour I see less than a few GB copied. This looks like it will take months to complete. TOTALLY UNUSABLE! What the *&*(* has Apple done here?