Sony “hid” a feature that may appeal to more demanding smartphone users, and it is something that was limited to Apple’s handsets and the Razer Phone.
Users have discovered that in addition to full HD and 4K modes – both at 60fps – the Xperia XZ Premium has a disabled mode that would allow images at up to 120 frames per second. The result of more frames in the rendering of the images would bring a more, fluid use, as well as a better experience in games and multimedia content, but unfortunately, it is not so easy to activate the feature. 
The company does not offer it natively, and in order to get it, it is necessary to migrate to a custom ROM that adds to the source code the script needed to activate the device’s third image mode. Is it worth the risk? In addition to losing security updates the company’s official system, many users are saying no, as the newfound still lacks improvements, and many Sony handset owners are reporting a maximum of 90fps in actual use.
Moreover, it is worth remembering that the quality of photographs of Xperia devices tends to fall with the use of modified systems, as line camera optimizations are protected by encrypted Sony keys that are inaccessible to unofficial systems.
Besides Xperia XZ Premium, basically all the devices launched by the company since 2016 are compatible with the 120fps mode: Xperia X, Xperia X Performance, Xperia XZ, Xperia XZ1, Xperia XZ1 Compact, Xperia XA2, and Xperia XA2 Ultra are other devices confirmed as fit.
So, what did you think of the discovery? Want to test?
Source: Xda-developers