calibre always must scan all the books on the device before registering it. Usually, it will cache the results of the scan in a file called metadata.calibre, on the device. That way calibre only has to look for new stuff.
T68 -- that's the e-ink tablet. Since it is a tablet, it probably has stuff all over the place in the file system. calibre has to scan all of that even if most of it isn't books. You can set calibre so only certain folders are scanned for books, from the device driver. Exclude the places where books never go.
"Configure this device" in the dropdown under "Device" (in the toolbar).