what's the most efficient way to load some insane numbers of lines into NS? Like in the 600k range...I can't imagine importing 60 files in

If its one time than csv import will be ok. If it is something that happens a lot, then Basically your own custom import. Building it is easy and comes in handy in lots of other cases as well. custom record > user event to show the button > calls the client script > calls the suitelet > calls the MR/Schedule script. Write the logic in MR/Schedule script for your import.

I'd probably look at using Celigo for this, or Celigo cloud extend

3932482 lines in total (including headers!!?!!)

Are you doing data migration from a legacy system ?

Nope, reimplementation. I think I'll argue for invoices only on anything closed, sod bringing in the SO's unless they're open

Uh... I wouldn't even bring that much over. Only unpaid open invoices.

Interesting...I hadn't considered that. Basically, import everything open/partial and drop everything else in a custom transaction (either AR or AP type) so it's at least retained for record keeping purposes

+1 for @KevinJ of Kansas approach of not brining in everything. For historical data the most common approach I've seen is the custom record approach. Uploads to custom records are generally faster as well. Celigo Data Loader (free) or Celigo CloudExtend Excel is an invaluable tool for migrations and can probably help you with the re-implementation as well.

I use to once a month update 1 million item records via Celigo. (don't get hit with the 25k chunk limit).