Found the cause.
sync_binlog was set to 1. I suspect that the default value is 5.5 was 0 and
that is has changed to 1
sometime after that.
Setting it to 0 boosted the performance back to normal (4x speed) and the HD
LED indicated much
lower stress on the hard disk.
Found this after I found
Hi Jørn,
Found this after I found out what caused it:
https://www.percona.com/blog/2009/01/21/beware-ext3-and-sync-binlog-do-not-play-well-together/
I suspect that this also apply to ext4, or?
I would go more specific and say that sync_binlog=1 does not play well with
single-threaded
