welcome to the forum,
no idea what programs are accessing the mount and how they access it.
tho, the answer would be in the debug log.
can use --dump=headers to see each api call.
one reason could be listing all those files over and over again
from the docs "The b2_list_file_names request will be sent once for every 1k files in the remote path"
might increase --dir-cache-time
and what is the reason for --vfs-read-chunk-size off ?
can check out my summary of the two vfs caches.
https://forum.rclone.org/t/status-about-using-rclone-for-music-storage-playback-in-2021-access-times-improved/27648/34