Not only does Qualcomm's Snapdragon 888 have a reputation for being extremely fast, but also for being very hot. Asus has already demonstrated that it can cool its smartphones excellently in the past, but the new chip poses a challenge even for the gaming king.
https://www.notebookcheck.net/ROG-Phone-5-Asus-is-only-partially-able-to-tame-the-Snapdragon-888.532693.0.html