If you’re using a relatively recent, non-Apple, sold-in-the-US smartphone, odds are good that it contains some kind of Qualcomm SoC. From 2011 or so up until now, the company has used a potent combination of LTE modems and custom CPU and GPU architecture designs to shove most of its early competitors (Texas Instruments, Nvidia, and Samsung) to the margins or out of the business entirely.

After a long string of successful flagships, the Snapdragon 810 has been a bit of a disappointment. At its best, it can be faster than the Snapdragon 800, 801, and 805 chips that it’s replacing. But the two 810-equipped phones we’ve used—HTC’s One M9 and LG’s G Flex 2—have definitely run hot, and performance slows down quickly as you use the phone.

Throttling processor speed in smartphones, tablets, and laptops to avoid overheating is completely normal, but the 810 runs especially warm. More interestingly, Samsung’s Exynos 7420—a chip which uses the same combination of Cortex A53 and A57 CPU cores at roughly the same maximum clock speed—is much faster than the 810, and it holds up better during extended use.

Now that we have a few phones in hand, we can quantify just how abnormal the 810’s heating issues are and talk about what an odd misstep it is from a company with an otherwise impressive mobile track record.

The test

The folks at Primate Labs have been working on mobile SoC throttling tests for Geekbench for a while now, and founder John Poole was kind enough to share the latest work-in-progress version of them with us for this piece.

Older versions of the test would slam all of an SoC’s CPU cores at once to generate as much heat as possible, but the new version attempts to simulate a more realistic usage scenario.

“The new thermal test only uses two cores,” Poole told Ars. “We could increase the test to use all available cores, but then 4- and 8-core designs would be doing two to four times the work of 2-core designs. The new thermal test also executes a fixed amount of work. Faster processors will be able to spend part of the test idle. This is similar to what most CPU-bound applications like games do on mobile.”

The test measures the CPU’s clock speed once every five seconds—the CPU can switch frequencies far more quickly than that, but this test still gives us a good broad overview of how the CPU will act over time given a sustained workload.

We first ran the test on three SoCs that we're pretty familiar with—the Snapdragon 800, 801, and 805. As the test ran, the 800 (in a Nexus 5) slowly and predictably dropped its clock speed from around 2.3GHz to around 1.6GHz. The 801 didn’t throttle at all, which isn’t entirely surprising—Snapdragon 801 phones generally had a very good mix of performance and battery life. And the 805 managed to run near its top-rated speed of 2.7GHz for quite a while, eventually settling down somewhere between 1.7GHz and 2.0GHz.

It’s a little more complicated to chart out the Snapdragon 810. When it throttles, it will occasionally switch from the “big” ARM Cortex A57 cores to the “little” ARM Cortex A53 cores. Even though the little cores could generally run at higher clock speeds than the big cores while the test was running, they can process fewer instructions per clock, making them slower overall.

The Snapdragon 805 is included for reference. A 2.0GHz Cortex A57 processor can be faster than a 2.7GHz Krait 450 processor if it's actually allowed to run at 2.0GHz for sustained periods, but the 810 throttles so quickly that the 805 and even the 801 can beat the 810 when performing sustained workloads. See our HTC One M9 review for more examples.

The gaps in the green line in the chart above (and the blue dotted line) indicate where the Snapdragon 810 in LG’s G Flex 2 switched from its big cores to its little cores. Not only does it need to hop to the little cores fairly often, but it almost never reaches its top clock speed of 2.0GHz. After just a few minutes, the big cores are reaching peak clock speeds of around 1.3GHz, and they eventually settle down in the 850MHz to 950MHz range when they’re not letting the little cores do the work.

Andrew Cunningham

Andrew Cunningham

The same SoC will perform a bit differently depending on which phone it’s in—software tweaks and the amount of heat the phone can safely dissipate are both major factors. That said, the HTC One M9 acts a lot like the G Flex 2—it rarely reaches its peak clock speed of 2.0GHz even in the hidden “high performance” mode that lets the SoC stretch its legs as it hops back and forth from the big and little cores. The big cores eventually spend most of their time between 1.2GHz and 850MHz. The M9 switches frequencies and cores more frequently than the Flex 2, but the chip is broadly acting the same way in both phones.

Now let’s look at the Exynos 7 Octa in the Galaxy S6. It has the exact same CPU core combination as the 810, but it can run at its top speed of 2.1GHz for a couple of minutes or so before throttling down. The SoC cools down a bit as its clock speed drops down, and the phone will let it jump back up to 2.1GHz briefly as there’s thermal headroom available. It almost never hops to the little cores—it takes more than ten minutes for it to happen at all, and it quickly jumps back to the big cores. Its minimum clock speed is about 1.2GHz, some 200MHz to 400MHz faster than the 810’s minimum speed. It’s a better-behaved chip all around.

Comparing the Exynos 7’s big cores to the Snapdragon 810’s big cores really drives the point home. On paper, these two CPUs should be near-identical in performance. In practice, the 810 throttles so early and so severely that even people who are just using their phones for a minute or two could run into slowdown.

For those who are interested, we've uploaded all of our raw data in CSV format here.

What it means

In short, chips throttle, but the 810 throttles more than most, and it's severe enough that the 810 is actually slower than the 801 or 805 in some CPU-bound tasks over the long haul. The Exynos 7 Octa, which has similar specs on paper, is much better in practice.

At this point, Qualcomm has implied to us several times that its use of ARM Cortex CPU cores was a stopgap measure—Apple got the 64-bit A7 chip to market around a year before anyone expected it to. Chips are designed over a period of two or three years, so using the ready-made Cortex cores were the quickest way to get a 64-bit response to market.

The results, unfortunately, don’t look great. It might be because the 810 is using a 20nm TSMC manufacturing process instead of the 14nm Samsung process used for the Exynos, or it might be that Samsung has more experience working Cortex CPU cores into its designs. Whatever the reason, our testing of real phones with these SoCs in them shows that 810-based phones are slower and have worse battery life.

Qualcomm's next major flagship is the Snapdragon 820, the first to use its custom-designed 64-bit “Kryo” architecture. Rumor has it that the chip will be made on the same 14nm Samsung process as the Exynos 7 Octa.

A return to its own CPU cores plus a newer manufacturing process should hopefully mean a return to the kind of performance and battery life we’ve gotten from Qualcomm-based phones in years past. All signs point to the 810 being a one-time slip-up and not the start of a trend—let’s hope that those signs are accurate.