An in-place upgrade and the culprit is MySQL binary log (we use it for replication).
Starting with MySQL 5.7 (Ubuntu 16.04) the default is sync_binlog=1, which syncs to disk with each write:
I switched to sync_binlog=0 temporarily, and OPL-update completed in 13 min. compared to 118 min. with sync_binlog=1.
hdparm reports 518.23 MB/sec.