Senior XDA member 'kdrag0n' has released the new Proton Kernel for the Asus ZenFone 6. Apart from bringing several enhancements, the Proton Kernel enables a 75 Hz display refresh rate option for smoother visuals. The Proton Kernel can be installed over the stock Asus ZenUI 6 Android Pie image after a bootloader unlock.
https://www.notebookcheck.net/Fancy-a-ZenFone-6-with-a-faster-display-refresh-rate-It-is-now-possible-with-the-custom-Proton-Kernel.434461.0.html