iPhone Duo reviews revealed varied opinions on its thermal performance. Some reviewers found the device merely warm, while others reported it becoming uncomfortably hot. Joanna Stern experienced excessive heat during FaceTime calls and photography, even needing to end a call due to the temperature. Apple explained this warmth as a byproduct of their thermal design to maintain performance. However, Stern perceived it as simply a hot phone.The iPhone Duo shares its vapor chamber and A20 Pro chip with the iPhone 18 Pro, but with a thinner vapor chamber and a graphite pipe in the hinge for heat transfer. The Verge noted significant heat buildup during extended camera use, especially with both displays active, leading to noticeable lag. PetaPixel also observed the device getting warm on the right side when performing processor-intensive tasks.Mrwhosetheboss found that the closed iPhone Duo trapped heat, resulting in lower performance compared to the iPhone 18 Pro Max. HTX Studio's tests showed the iPhone Duo reaching higher temperatures than iPhone 18 Pro models during gaming and experiencing reduced frame rates and brightness. While 9to5Mac stated heat didn't impact performance, Tom's Guide found it slightly hotter than a comparable Samsung device. Conversely, Gizmodo reported the vapor chamber effectively prevented lag. The public will soon see if heat becomes an issue for everyday users.