Initially, the process was taking a few seconds, but that was only a subset of the data. Once I got this larger set of data, it was taking 18:49 to import it, which wasn't acceptable. So, time to leave the straight "insert into ___ set ___=___" method that is so much more readable and move to "insert into ___ values ( __ , __ )" style.
The first thing I did in testing was to disable indexes, insert, then enable indexes again. This brought the time down to 9:27, which is a big improvement, but still not where I wanted to be.
Changing from set _=_ to values ( _ , _ ) made no real difference.
Then, I changed to doing multiple records in a single pass, and what a difference that mad…