Comment by electroly

This is the smoking gun, unfortunately; I was right. This code is not correct. You're doing a single insert per transaction (which is the same thing you'd get if you hadn't used an explicit transaction at all). What you want is a single transaction with ALL of the inserts in it. Do it again with a single transaction and you'll find it massively improves the performance. This is a common pitfall for beginners.

Edit: You've changed the link after I wrote this reply. Did you fix the code? Is the blog post using the original code or this updated code?