That's really slow, especially for documents with that few of lines. Is there a bunch of scripting/workflows running on those document types? Is it all still needed?
That could slow it down. You could try a small import with that box unchecked saying "run scripts and workflows on import" to see if that helps the speed. Then if those scripts/workflows are doing something critical, you'd have to manually do that or delete the records and re-import with that box checked. If that fixes the issue, then you'd have to figure out what about the scripts/workflows is slowing it down so much.
If that's not the issue, yeah I'd probably complain to NetSuite. I don't know if this is still supposed to be the case, but I thought the stated potential speed on CSV imports was like 6 per second, though I've never come close to getting that. 8 and 17 seconds is way slow though if there are not automations slowing down the record saves.