Paypal only replaced their web servers/backends, they didn't replace "real" backends the web servers communicate with and that handle the actual business logic/transfer of money etc. And the devs from paypal later admitted that the Java systems they maintained had problems of their own (which were not caused by java but rather by bad programming by themselves stacked over time) so switching to node was partly an excuse for the management to allow them to rewrite large parts of the systems. If they started fresh with java, i guess the performance would be at least the same, if not better (due to how java natively supports multithreading)