Someone from oracle posted here a while ago, where they described Oracle 12.2 has 25 million lines of C https://news.ycombinator.com/item?id=18442941
An explosion on the number of lines of code is one of the way development teams fail.
The truth is that Oracle is ridden with coherence bugs, and have a much worse performance picture out of the box than Postgres. But while improving the Postgres performance requires deep digging into the DBMS itself, Oracle has a lot you can optimize just on the SQL interface. But there are plenty of cases where Oracle just can not become as fast as out of the box Postgres, and many where it could in theory, but a bug prevents you from doing things right.
Overall, three is no definitive ordering on the speed of the "good 3" DBMS. It always depends on what you are doing.