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Flutter jank means a frame missed its display deadline. To fix it, reproduce the stutter in a profile build on a physical device, use Flutter DevTools to determine whether the UI thread or raster thread is late, then change the code or rendering work implicated by that frame trace. A debug build, a single average, or a guess about expensive widgets can point you in the wrong direction.
What causes jank in Flutter?
A display updates in frames. Flutter’s guidance uses an approximate frame budget of 16 milliseconds at 60 Hz and 8 milliseconds at 120 Hz; the relevant target depends on the device’s refresh rate. These are timing budgets, not a guarantee that an app will achieve a particular frame rate. When a frame takes too long, motion may look jerky or appear to skip.
Flutter divides frame work between two threads. The UI thread runs Dart and framework work and creates a layer tree. The raster thread renders that tree through the engine and GPU. Either side can miss the deadline, so begin by identifying which side is slow rather than optimizing by symptom alone. Flutter’s Performance view documentation explains the frame chart and its timing guidance.
Measure the interaction under representative conditions
First reproduce the exact action that stutters: for example, a particular scroll, route transition, animation, or first-use effect. Performance timing from a debug build is not representative because debug mode adds costs and behaves differently. Use profile mode for analysis, and test on physical Android or iOS hardware rather than assuming an emulator or simulator has the same characteristics. If the issue is device-specific, include a physical device near the low end of the range your app supports. See Flutter’s build-mode guidance and its profiling guidance.
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- Build and run the app in profile mode on a physical target device.
- Open Flutter DevTools and select the Performance view.
- Repeat the interaction that reliably produces the stutter, then inspect the corresponding frames.
- Keep the same device and interaction when testing a change, so the before-and-after timing is comparable.
Use the frame chart to locate the bottleneck
In DevTools’ Flutter frames chart, each frame shows paired UI-thread and raster-thread timing bars. Select a slow frame to update Frame analysis and the timeline; inspect the events around the overrun before editing code. A UI-side overrun points toward synchronous Dart or framework work, rebuilds, layout, or paint preparation. A raster-side overrun points toward the cost of rendering the scene. The chart helps narrow the investigation, but a single slow frame does not by itself establish the root cause.
The Performance view can mark shader-compilation frames in dark red. It also provides layer toggles for investigating effects such as clipping, opacity, and physical shapes. Treat these as diagnostic experiments: reproduce the same interaction with a toggle changed and compare the relevant frames. The evidence should determine whether a given effect matters in your scene, rather than a blanket rule against visual effects.
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Fix the kind of work the trace identifies
If UI-thread work is late
- Find repeated or unexpectedly broad rebuilds. Track widget builds during the affected interaction. Avoid expensive operations repeated in
build(), and split large widgets around meaningful encapsulation and change boundaries. A call tosetState()rebuilds descendants of that state object, so inspect how much of the subtree it affects. Flutter’s performance best practices cover these patterns. - Keep static subtrees out of repeated animation work. If an
AnimatedBuilderbuilder runs repeatedly, construct animation-independent content once and pass it through the builder’schildparameter instead of recreating it each time. - Use lazy construction for large collections. When a long list shows only part of its contents at once, prefer a lazily built
ListViewover eagerly creating a large collection of children. See Flutter’s rendering performance guidance. - Move blocking computation off the main isolate when warranted. Isolates are appropriate when large computations take longer than the frame gap and block frame work. Large data parsing, filtering, and media processing are examples to investigate. Do not move work to an isolate automatically; use the trace to establish that synchronous computation is causing the delay. Flutter explains this in Concurrency and isolates.
If raster-thread work is late
Inspect the selected frame’s rendering events and the scene being drawn. Flutter calls out unnecessary saveLayer() work and some combinations of opacity, clipping, and shadows as possible costs. Use the Performance view’s layer toggles to test whether the relevant class of effect contributes to the raster delay, then simplify only the parts shown to matter. A rendering effect is not automatically a problem just because it can be expensive in some scenes.
If only the first mobile animation stutters
First-run behavior differs from repeated-run behavior, so compare both in the same profile setup. Flutter’s rendering guidance advises using its default graphics renderer, Impeller, for noticeable jank confined to the first run of an animation. Check the target platform and Flutter version before applying renderer-specific advice, and consult Flutter’s Impeller guidance.
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If the target is Flutter web
Use Chrome DevTools’ Performance panel to inspect Flutter web timeline events rather than relying on the Flutter mobile and desktop frame chart. Flutter documents optional flags that add build, layout, and paint events; its web profiling guidance says profiling web apps requires Flutter 3.14 or later. See Flutter’s web performance documentation and Chrome’s Performance panel guide.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Verify the change with frame timing
After changing the suspected cause, repeat the same interaction in profile mode on the same physical device and compare the affected frames. For ongoing tracking, Flutter identifies buildDuration, rasterDuration, and totalSpan as frame timing metrics. Its metrics guidance recommends looking at the average, 90th percentile, 99th percentile, and worst build and raster times. Percentiles and worst-case values can expose intermittent or tail slowdowns that an average hides. See Flutter’s performance metrics guidance.
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- Compare UI and raster timing separately to see whether the change addressed the measured bottleneck.
- Compare the same interaction and device before and after the change.
- Check first-run and repeated-run behavior separately when the symptom occurs only on first use.
- Use averages alongside the 90th percentile, 99th percentile, and worst frame rather than treating one summary number as decisive.
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