welcome to the forum,
not sure the exact reason for the high memory usage.
i use mount+webdav, windows and linux, never had issues with high memory usage.
that flag does nothing, as it was used in the deprecated cache remote.
that is for the vfs file cache which is stored at vfs cache: root is "C:\\Users\\tom\\AppData\\Local\\rclone"
so that has nothing to do with memory.
rclone mount has two caches, vfs dir cache uses memory, vfs file cache uses local storage.
check out my summary of the two caches:
https://forum.rclone.org/t/status-about-using-rclone-for-music-storage-playback-in-2021-access-times-improved/27648/34