FWIW you still haven't explained how the upgrade was performed.
That might be a very important piece of information, because the
9.4 cluster might have hint bits set and/or the data may be
mostly frozen, but the 9.6 cluster may not have that yet,
resulting in higher CPU usage.
We upgraded the database with a pg_dumpall from 8.4.22 and then a psql < BACKUP.SAV command!
Not to miss the obvious: did you run ANALYZE after importing the backup?
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