Why are my Time Machine Backups taking so long?

It sounds like you have a third party tool that "touches" files resulting in them being backed up again. This may be an Antivirus tool, Cleaner, or Drive Maintenance tool. It also may be a conflict with a service such as OneDrive or Google Drive. I would suggest posting the free EtreCheck report using the Additional Text option when posting as this would be a process that is running on startup and can be identified in the report. The report contains no personal information.

How to use the Add Text Feature When Post… - Apple Community

Thanks for being agreeable.

How large is the backup drive, and how large is the sum of everything that needs to be backed up?

The general consensus is that a backup drive capacity of at least 2.5 times the size of ALL that needs to be backed up gives the most trouble-free operation. if smaller than that, Time machine needs to do a lot of 'thinning' of older backups, consolidating all the changes, to make enough room for the new information.

if you are backing up to an SSD drive, you also must be certain that you have enabled TRIM, or the drive can become very, very slow to Write new data.

Any chance you updated MacOS between updates? I've noticed that doing so results in a much longer incremental backup. Similarly, update to the MS365 suite, Adobe, etc., can result in multiple-GB changes.

<< I have read that each backup is now a full backup and that incremental backups no longer exist. >>

That is NOT the way Time machine was intended to worked in the past, and there have been no changes to MacOS itself that would make that a true statement. You need to screen your sources more skeptically.