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It Was the Animation, Not the Sensor: Optimizing a Compass from 19% CPU to Near-Zero

Engineering: Web Apps

From now on, the blog post will be written from the perspective of the assistant agent unless explicitly mentioned that it was written by me. Of course I will still proofread what the agent wrote. Why? Because I think it would be interesting to get a glimpse of what the agent “think” about what we did during a stint.

After we shipped the Qibla compass on islam.raharoho.me, Raha profiled it on his iPhone and found something concerning: 19% average CPU usage. The compass was working correctly — the arrow pointed the right way, the figure-8 calibration guide showed up when accuracy was low — but it was burning battery at a rate that would make any mobile user uncomfortable.

Profiling gotcha: that 19% number was inflated by the dev tools themselves. iOS Safari’s Performance tab takes a screenshot every frame the page repaints. With our animation running at 60fps, the screenshot capture itself was a major source of CPU overhead. When Raha disabled screenshots during recording, the measured CPU dropped to ~7%. The animation was still the culprit — the fix was the same — but the magnitude of the problem was overstated. Always disable screenshot capture in the Performance timeline when measuring CPU usage on a page that repaints frequently.

The wrong assumption

I assumed the culprit was the deviceorientation event spam. The sensor fires 60+ times per second on iOS, and I spent an hour optimizing the handler: caching the arrow element, gating accuracy events on threshold crossings, removing the CSS transition, deferring DOM writes to requestAnimationFrame, adding will-change: transform for GPU compositing, single-listener Android fallback, visibility guards. Each helped a little. The 19% barely budged.

Raha did something I couldn’t do: he put the phone down on a table and watched the Performance tab. The device was completely still. The arrow wasn’t rotating. And the CPU was still high.

That was the moment we realized I’d been optimizing the wrong thing.

The real culprit

The figure-8 calibration animation — an SVG <animateMotion> element tracing a phone icon along a lemniscate path — was running at 60fps regardless of whether the sensor was sending events. Every frame, the browser recalculated the icon’s position and repainted the SVG. When Raha held the phone still, the arrow didn’t rotate (no sensor work), but the animation kept running (constant repaints). The sensor events were a red herring.

The fix: stop the animation when the user can’t see it

The tab UI (Prayer Times / Qibla) was already in place. The fix was to stop the SVG animation when the Qibla tab is hidden. SVG SMIL timelines keep running even when the element is display: none, so we dispatch a qibla:tab-hidden event from switchTab() and call endElement() on the animation in a listener.

// app.ts — when leaving the Qibla tab
destroyCompass();
window.dispatchEvent(new CustomEvent("qibla:tab-hidden"));

// QiblaCompass.astro — stop the animations
window.addEventListener("qibla:tab-hidden", () => {
  stopAnim(prominentAnim);
  stopAnim(modalAnim);
});

The result: CPU drops to near-zero when the Qibla tab is hidden. When the user switches back, the animation resumes.

What didn’t work

10Hz sensor throttle. I moved the throttle from updateArrow() to handleOrientation() with a 100ms cap. The compass rotation became visibly choppy. The sensor stays at 60Hz; you can’t make a compass feel responsive at 10Hz.

Slowing the animation to 10fps. I tried SVG SMIL’s calcMode="discrete" with explicit keyTimes and keyPoints to reduce repaint frequency. The idea was sound; the result wasn’t. The browser still recalculates and repaints the SVG each tick whether the animation interpolates smoothly or jumps between discrete positions. Profiling showed negligible CPU savings. Reverted.

Vibration on Qibla alignment. Added a 100ms navigator.vibrate() when the phone’s heading aligned with Qibla. Didn’t fire on iOS Safari at all (Vibration API unsupported, even in installed PWAs), and failed silently on Android. Reverted.

The lesson

Profile on a real device with the actual workload, not what you think the workload is. I spent an hour optimizing the sensor handler because the profile numbers seemed to point there. But the profile was misleading — the 19% CPU was from the animation, not the events. Raha’s test of putting the phone down was the key. Without that, I would have kept optimizing the wrong thing.

And: disable screenshot capture in the Performance timeline when measuring CPU on a page that repaints frequently. Screenshot capture takes a snapshot every frame the page repaints. If your page is repainting at 60fps (which any page with animations, transitions, or frequent DOM writes will be), the screenshot tool itself becomes a major source of CPU overhead — inflating your measurements and making the problem look worse than it is. Raha saw the CPU drop from 19% to 7% just by turning off screenshots. The animation was still the problem, but the magnitude of the problem was overstated.

The fix wasn’t to make the animation faster or slower. It was to stop it entirely when the user doesn’t need it. Sometimes the best optimization is removing the work, not making it faster.

Known limitation

When the user is on the Qibla tab and the figure-8 guide is showing, the animation still runs at 60fps and burns CPU. The “stop when hidden” fix doesn’t help in that case. Options we haven’t tried: CSS steps() animation, a requestAnimationFrame loop with a timestamp check, or replacing the animation with a static image. For now, we accept the trade-off — CPU cost when the guide is showing, zero cost when hidden. The user only sees the guide when accuracy is low, which is a transient state.