Doom likes to run fast, and especially so with a GTX 1080 pushing pixels. We figured the game's OpenGL mode would be an ideal test of each CPU's ability to keep that beast of a graphics card fed, so we cranked up all of its eye candy at 1920x1080 and went to work with our usual test run in the beginning of the Foundry level.
The nice progression of average frame rates above suggests our hunch about the GTX 1080's hunger for work from the CPU is correct. The fast clocks and high IPC of the Core i7-6700K and Core i7-7700K result in the best performance from the GTX 1080, and we bet the extra bandwidth afforded by the DDR4-3866 memory we're using doesn't hurt, either. At the other end of the chart, pairing the FX-8370 with the GTX 1080 cuts the card's average frame rate roughly in half compared to the Core i7-7700K's. Ouch. The AMD chip's 99th-percentile frame time is significantly higher than those turned in by the Intel chips, too.
These "time spent beyond X" graphs are meant to show "badness," those instances where animation may be less than fluid—or at least less than perfect. The formulas behind these graphs add up the amount of time the GTX 1080 spends beyond certain frame-time thresholds, each with an important implication for gaming smoothness. The 50-ms threshold is the most notable one, since it corresponds to a 20-FPS average. We figure if you're not rendering any faster than 20 FPS, even for a moment, then the user is likely to perceive a slowdown. 33 ms correlates to 30 FPS or a 30Hz refresh rate. Go beyond that with vsync on, and you're into the bad voodoo of quantization slowdowns. 16.7 ms correlates to 60 FPS, that golden mark that we'd like to achieve (or surpass) for each and every frame. And 8.3 ms corresponds to 120 FPS, an even more demanding standard that Doom can easily meet or surpass on hardware that's up to the task.
Happily, none of the chips in our test suite cause the GTX 1080 to spend more than a handful of milliseconds beyond the critical 16.7-ms barrier. The Core i7-6700K and the Core i7-7700K each let the GTX 1080 spend less than half a second working on frames that take longer than 8.3 ms to produce, however, compared to the two to five seconds that the Sandy, Ivy, and Haswell Core i7s produce. The FX-8370 and GTX 1080 combo spends more than 12 seconds under 120 FPS, though, a disappointing result in this company.