GPU junction temperatures are meant to be between 70-90°C on these air coolers. Thermal throttling should never occur unless something is seriously wrong. The card is designed to maintain peak performance at all times, while staying within the power and thermal limits. When a card thermal throttles it is exceeding its thermal limits, and must reduce its performance to maintain stability. Shutoff temperature is at 115°C, your card will actually just turn off to prevent damage when it reaches that temperature.

Essentially, yes. It's a protection mechanism for the GPU to prevent overheating, so it slows down to reduce temperature. Stuttering can have other causes though, so it's just an indicator that thermal throttling may be occurring. To actually validate it, you need some way to monitor the GPU temperature under load. You can enable an overlay in Radeon Software that shows your hardware stats during gameplay, or use some other software like GPU-Z or HWInfo.

Radeon Software has hardware monitoring built in. If your junction temperature reaches 110 degrees C, your card is thermal throttling. When this happens, the clock speed of the GPU gets cut back significantly which shows as stuttering in gameplay.

We've hit a point of diminishing returns with current graphics. We are very good at approximating the majority of real world effects at almost no cost to performance. But getting those last few effects is what's hard right now. Ray tracing is not only more computationally intensive, it also just hasn't had that much research done for optimising it for performance. All the ray tracing engines you see in 3D video production software are optimised for accuracy and flexibility. We don't just need more powerful hardware, we also need to figure out how to better utilise it for these new techniques, which takes time.

Fair point

Isn't this a GearVR without the phone in it?