Snapdragon 810, right from the day of the announcement has been under a fair bit of stress and pressure. The coveted and much looked forward to SoC has been under the keen observation especially cause of the temperatures that it can reach when the device is put under extreme stress. It is reported that the same problem had occurred while Samsung was testing a Snapdragon 810 variant for their Galaxy S6 and S6 Edge range of devices, and this is precisely why they went ahead with the Exynos variant itself rather than the usual two sets of models, where in America and Europe, they go with the Snapdragon devices while Asia and the rest get the Exynos one.
A Dutch website seems to have validated the claims of everyone who said that Snapdragon 810 gets really hot as they have posted the temperature scales which show that the HTC M9 running the SoC goes as high as 130 degree Fahrenheit which is roughly 56 degree celsius. The temperature was measured using a temperature gun which was marked when running the highly intensive GFXBench benchmark application.
In comparison, the likes of LG G3 reached a temperature of 42.2 degree celsius while the iPhone 6 Plus clocked at 39.4 degree celsius. The HTC M8, the previous flagship performed much better with temperature of 38.7 degree celsius.