Every import will request all the data for the entire journal from LJ (for comparison, in case there are entries or comments that exist on the LJ side but are missing on the DW side), but the amount of time it takes for DW's servers to process the data, post-download, will vary based on how many entries/comments need to be processed and how many are just a quick "yup, got this one in both places, it's all good". Unless you're dealing with very large journals, though, the difference is pretty negligible.