Why do some write sometimes go faster than expected for MEDIUMTEXT
I suspect the answer goes something like this...

Approach 1:  800KB takes a lot of disk activity to write.  This takes perhaps 1000ms.  When you "wrote" a row in 31ms, that was because of most of the I/O being delayed until after acknowledging the action.  When it took 718ms, the disk was busy writing other things, so it was slow at accepting the current row.

Approach 2:  A rule of thumb for I/O with a HDD is 100/second.  800KB = 50 * 16KB blocks to write (plus some overhead).  That would compute out to 500ms per row.  So 718ms sounds a little high, and 31ms sounds like a lot of caching going on.

So the question should be "Why does this write sometimes go faster than expected?