The Elder Scrolls V: Skyrim
Our Skyrim test involved running around the town of Whiterun, starting from the city gates, all the way up to Dragonsreach, and then back down again.
We tested at a 1080p resolution using the "Medium" detail preset. (The "High" preset was playable on the GT 640, but it wasn't completely smooth.)
Now, we should preface the results below with a little primer on our testing methodology. Along with measuring average frames per second, we delve inside the second to look at frame rendering times. Studying the time taken to render each frame gives us a better sense of playability, because it highlights issues like stuttering that can occur—and be felt by the player—within the span of one second. Charting frame times shows these issues clear as day, while charting average frames per second obscures them.
For example, imagine one hypothetical second of gameplay. Almost all frames in that second are rendered in 16.7 ms, but the game briefly hangs, taking a disproportionate 100 ms to produce one frame and then catching up by cranking out the next frame in 5 ms—not an uncommon scenario. You're going to feel the game hitch, but the FPS counter will only report a dip from 60 to 56 FPS, which would suggest a negligible, imperceptible change. Looking inside the second helps us detect such skips, as well as other issues that conventional frame rate data measured in FPS tends to obscure.
We're going to start by charting frame times over the totality of a representative run for each system—though we conducted five runs per system to sure our results are solid. These plots should give us an at-a-glance impression of overall playability, warts and all. (Note that, since we're looking at frame latencies, plots sitting lower on the Y axis indicate quicker solutions.)
The GT 640 exhibits much higher and more frequent latency spikes than the Radeon HD 7750 and the GeForce GTX 550 Ti in our Skyrim test. It also produces less consistent frame times overall (as evidenced by the fatter-looking plot), which is exactly the opposite of what we want.
We can slice and dice our raw frame-time data in other ways to show different facets of the performance picture. Let's start with something we're all familiar with: average frames per second. Though this metric doesn't account for irregularities in frame latencies, it does give us some sense of typical performance.
Next, we can demarcate the threshold below which 99% of frames are rendered. The lower the threshold, the more fluid the game. This metric offers a sense of overall frame latency, but it filters out fringe cases.
Of course, the 99th percentile result only shows a single point along the latency curve. We can show you that whole curve, as well. With single-GPU configs like these, the right hand-side of the graph—and especially the last 5% or so—is where you'll want to look. That section tends to be where the best and worst solutions diverge.
Our raw frame-time plots show several big spikes above 80-ms for the GT 640, and those are reflected in the latency curve.
Finally, we can rank solutions based on how long they spent working on frames that took longer than 50 ms to render. The results should ideally be "0" across the board, because the illusion of motion becomes hard to maintain once frame latencies rise above 50-ms or so. (50 ms frame times are equivalent to a 20 FPS average.) Simply put, this metric is a measure of "badness." It tells us about the scope of delays in frame delivery during the test scenario.
No matter how you slice it, the results are clear. In Skyrim, the GT 640 isn't just slower than the Radeon HD 7750 overall; it's also worse at keeping frame times consistent, which means it doesn't do as good a job of maintaining the illusion of motion.
From a seat-of-the-pants perspective, Skyrim doesn't feel completely fluid on the GT 640. While the game responds well to input, movement that should be smooth is punctuated by skips at short and regular intervals. Perhaps the GPU and its anemic memory configuration are to blame, or maybe it's just a driver optimization issue. Either way, playing Skyrim on the GT 640 just isn't a very good experience, even at this rather low detail preset.