Is my phone throttling? How to tell from a benchmark

Throttling leaves a signature: speed that drops across a run rather than staying flat. Here is what the Speed Under Load chart shows, what the throttling flag actually means, which other numbers move with it, and what a browser cannot tell you.

Throttling has a signature, and it is not a temperature. It is speed that falls across a run: a device that starts a task fast and finishes it slower than it began. A benchmark that measures the same work repeatedly, for long enough, can see that happen — and that is the only way a browser can see it at all, because there is no way for a web page to read a processor's temperature.

The chart to read

On the results screen, Speed Under Load plots how much of its starting speed the device held, second by second, through the processor test. It is built from the work each core got through in every chunk of the loop, compared against the first chunks. A device that is not throttling draws a flat line near the top. A device that is throttling draws a line that sags — sometimes gently, sometimes in a step when the processor drops to a lower clock all at once.

The shape tells you more than any single number. A slow fade over the whole run is heat building up. A sharp step partway through is a power limit being applied. A line that starts low and stays low is not throttling at all — the device is simply slow, or it was already hot before the run started.

What the throttling flag actually means

The word Throttling appears on the screen when a chunk of the processor loop takes more than a quarter longer than the first ones did, after enough chunks have run to know what the starting pace was. That is the whole rule. It is a plain comparison of the device against itself a few seconds earlier, not a reading from a sensor, and it can be wrong in both directions: a background app starting up can trip it on a cool device, and a device that was already throttled before the run began will never trip it, because there was no faster pace to fall from.

The other numbers that move with it

• Pass spread. Each subtest runs three times, and the caption under the score says how far the passes differed. On a throttling device the later passes are slower than the first, and the spread widens. On a device holding its speed the three agree closely.
• 1% low FPS. When a device slows mid-run, the slowest frames get slower first. The average can stay respectable while the 1% low falls away from it — the same gap that makes a device feel worse than its average suggests. There is a separate guide to that number.
• Core structure. If one worker in the processor test finished far behind the others, that is usually a big-and-little chip doing what it is designed to do, not throttling. The spread between workers is shown so the two are not confused.

What a browser cannot tell you

How hot the chip is. Which part of it is hot. Whether the slowdown was thermal, or a battery-saver mode, or the operating system deciding another app mattered more. All of those produce the same shape on the chart. This site shows the shape and names it throttling, which is what it is, and stops there. Earlier versions of this project displayed a temperature — simulated, with randomness added so it looked measured — and that was deleted rather than replaced.

What to do with the result

If the line sags, the device was slowing itself down during the run. Whether that matters depends on why. A phone that has been on a shop display all day, or one that has just spent an hour on a game, will throttle and recover. A phone that throttles after a minute from cold, every time, is telling you something about its cooling or its condition. The way to tell those apart is the same as for any measurement: let it cool, run it again, and compare. The guide on what makes a device throttle covers the causes and what changes them.

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