Thought I uninstalled the old plugins from v5, guess not. I did have apcupsd and unmenu running, disabled unmenu and booted into safe mode, showing 1.1MB/s now, current position 533MB after 5 minutes, going to let it continue to run.
EDIT: Think i might've found my problem. Went thru and read a large file from /mnt/disk* for each drive with dd, all drives came back with 100+ MB/s except one. That drive is dying isn't it?
That fixed it, swapped drives around now rebuilding at ~110MB/s. Guess I had a drive decide to fail at the exact same time I decided to try beta6. Thanks for the help jonp