Sony's Xperia Z3+ known as the Xperia Z4 in Japan is the most recent offering from the company with flagship worthy features. It represents an incremental upgrade to its predecessor with only a few hardware design changes and some internal improvements like the front-facing camera and the Qualcomm Snapdragon 810 processor. This processor has its share of bad reviews, even before it was launched there were rumors that there were some issues with the way the processor handled its temperature thus delaying the production. LG was one of the first companies to use this processor denying these rumors along with Qualcomm, but reports about the HTC One M9 overheating made the rumors emerge once again.
Samsung decided to use their own Exynos 7420 in their newest flagship, the Galaxy S6, one of the possible reasons could be that the company wanted to have the least amount of controversies as this device was extremely important to recover some of the sales that previous generations had not been able to achieve. The phone is a success as reviewers praised the new design that included better materials than the plastic used in its predecessors, the screen is considered one of the best out there and its camera takes great pictures in various situations.
Speaking of cameras, 4K video recording is becoming a standard feature in high-end offerings, both the Galaxy S6 and the Xperia Z3+ include it. Even when the latter handset uses a new version of the Snapdragon 810 processor called v2.1, there have been reports that the temperature rises when using some demanding apps, in particular, the camera although an update from Sony is expected to solve this to some extent. The Galaxy S6 only allows to take 5-minute clips in 4K resolution as this files can get pretty large (7 minutes can take about 2 GB) and let's not forget that this handset doesn't support expandable memory. The Sony Xperia Z3+ only restricts the length of the video by closing the app if the temperature gets too hot.
Now, there's a video from a YouTube user that shows that the temperature of the processor after 5 minutes of 4K video recording on the Galaxy S6 of about 70°C is actually higher than the one in the Xperia Z3+ of about 50°C after 6 minutes of recording video at the same resolution, although some sensors reached a temperature of 66°C. The app that was used to measure the temperature was CPU-Z and let's keep in mind that temperatures may vary in other conditions such as under direct sunlight or in a warmer room, but at least in the video, the overheating issue of the Xperia Z3+ is not as bad as some reports suggest and it certainly doesn't seem to affect how the device works.