https://wccftech.com/ios-15-4-allow-third-party-apps-take-advantage-of-120hz-promotion-technology/

Apple introduced 120Hz ProMotion technology for the first when it launched the iPhone 13 Pro and iPhone 13 Pro Max last year. While this meant users finally had access to smoother scrolling and browsing on an iPhone, there were some issues that still needed to be addressed. For instance, third-party app animations were limited to 60Hz and were unable to take advantage of that 120Hz refresh rate. Apple later stated that there was a Core Animation bug that would be fixed, and thankfully, that day has finally arrived.

The transition from 60Hz to 120Hz for third-party applications was noticed by Apollo developer Christian Selig, who states on Twitter that he received a message from someone working at Apple that a fix had been issued in the iOS 15.4 update. He was later able to confirm the change himself and sounded pretty excited over it too. One thing he does notice is how the animation is not quite like the iPad’s ProMotion technology, and it requires a little tweak. He mentions this little detail below.