Problem with Maintenance of Library

I manage all my moves and series using the TinyMediaManager program, and all the files have preview files included. In the Mezzmo settings I have video metadata retrieval set to use preview files if available (and it is for 100% of the files), but any time I run the maintenance process, it takes a lot of time to finish, ffmpeg process is running and Mezzmo System process says that it is rebuilding preview images. After the process is done, I still see previews I have as images, so why maintenance uses ffmepg for previews, and why it takes soo much time (for 10.000 files, it takes 8-12 hours on my 4C/8T Ryzen 5 CPU). How can I disable this process in the first place and have Mezzmo rely on the external preview files only?

The preview thumbnail images are for seeking within a video on supported devices like the Roku and web access. You can uncheck the box for this in the Maintenance dialog to reduce the time maintain library takes.