Rocket League never ran badly on the PlayStation 4, but it did often drift from its baseline 60 frames per second in moment of chaos.
Now it doesn't.
Post the release of Rocket League on the Xbox One, Digital Foundry took a comprehensive look at the performance of Rocket League on both the Xbox One and the PlayStation 4. They discovered that, as a result of post-launch patches, the performance of Rocket League on the PlayStation 4 had improved markedly.
Interestingly, the Xbox One version runs pretty much identically. A solid 60 frames per second at all times with next to no dips in frame-rate.
It's the sort of thing you're unlikely to notice in the heat of the game, but I can't help but get wrapped up in this sort of tech-geekery.
Worth a watch if you're as pedantic as me!