is it possible to increase the number of tasks being executed at once?
Under options - connection - all i have is number of attempts and time between retry. There's no option for threads.
It's somewhat moot now since it finished deleting everything. Although would still be nice to know how to change that for the future.
As for your suggestions as to which storage to use - glacier was the correct one. This was just an old vault that I no longer needed and hadn't gotten around to deleting it since it's somewhat of a pain to delete a vault. And these are files that if I lost my local copies, I would only need to restore a small number of them quickly - the rest could be restored at the slower/cheaper glacier rate.