This is normal, but throttling does occur at stock configuration. That's why I spent a few days fiddling around with Wattman then checking performance and stability through 3DMark, and when I found that I couldn't stand the incessant high-pitched whirring of the blower fans, replaced the stock cooler with the aftermarket Morpheus II + 2 case fans.
Now my temps are at least 15 degrees lower with better performance overall.