For a project a couple of years ago, I used Go language and what I did was a very simple thing: I took advantage its channel mechanism and created a slice to save (memoize more or less) 100K generated data which I inserted with a BEGIN TRANSACTION / END; in other words, every 100K I would insert them once and repeat.

If my memory serves me well, in approximately 6.8 seconds (give or take), I could insert 10M rows and bear in mind I didn't use WAL at all.

Not bad I would say, not bad at all.