That's hot. Really, really hot: considerably higher than iPhone 6 Plus, LG G3, Galaxy Note 4, and last year's HTC One M8, all of which hovered around the 40 degree Celsius mark (104 Fahrenheit) under the same test. 55 degrees is certainly hot enough to be uncomfortable in your hand, and possibly dangerous during extended use. For the sake of comparison, that's the temperature of a high-end desktop GPU under the stress of an intense, high-performance game - and those cards are designed to take that heat, with custom enclosures, heatsinks, and fans to channel hot air away from the computer. (Not to mention the fact that desktop GPUs don't sit millimeters away from lithium-polymer batteries.) Mobile phones have to rely on passive thermals to shed heat - that's the phone case and your hand, if you're wondering. Strangely, the phone did not automatically shut down while running so hot.
Though infrared temperature measurement isn't as exact as other means, it's hard to deny that the new chipset is running much, much hotter than comparable phones; the HTC One M8 has a similar metal case design and uses the Snapdragon 801. Tweakers reports that the phone doesn't get nearly so hot when using applications that are less intense, including mobile games, though they do note that it's still considerably warmer than other phones. The test was also performed with "non-final software."