The –disable-filetime-check documentation doesn’t say what it really does, but the code has this opinion:
which (if correct) means it will make things worse. Does File Explorer show any recent modified dates?
You can find exactly what it thinks about a given file if you use verbose logging to show file exam results.
Backups take very long with large data sets shows sample output and how you might set up the logging.
Does it show files going by in the GUI? If not, your I/O might be something other than it reading your files.
For a high-tech way of seeing what it’s actually doing on disk, you can use Sysinternals Process Monitor.