Interstingly, if you're seeing filenames scrolling by (I assume you specified the v option to tar, as well), then the tightest bottleneck for the tar process is likely to be waiting for your display.

If you really want to capture those filenames somewhere, then I'd suggest that you redirect the output to disk by placing a ``> /mnt/linux-bx.txt'' at the end of the tar command line. It might even be worth your while to break out of the current backup (by pressing Ctrl-C, in all probability) and start over with that redirect (or not specifying `v'), depending on how many files you're backing up.