I have just discovered Darktable, which is an integrated workflow system that (a) groks ubuntu quite well and (b) is opensource. It has a lightbox and file manager, handles the exports I like to do, and seems to be quite easy to work with. Has good reviews.
Early notes: it exports .tiff fairly rapidly: including turning everything on the card (there are 309 pictures there at present) both ,jpg and .nef, into .tiffs. And it can then export selected files, allowing you to set size.
This allows for a much faster workflow for me.