Capacitor Performance

Find the bottleneck first (WebView JS, bridge, native, or network), then fix the specific cause. Generic web advice (code splitting, image compression, virtual lists) still applies; this skill focuses on what is different inside Capacitor.

When to Use

TRIGGER when:

Cold start is slow, splash stays long, or the first screen is blank for seconds.
App "restarts" or shows a white screen after taking a photo, opening a big file, or returning from background.
Native calls feel slow, especially with images, files, or large HTTP responses.
Scrolling/animations jank only on device (often older Android WebView).
Memory grows over time; listeners pile up.

Do not use:

Crashes, native stack traces -> debugging-capacitor, capacitor-native-observability.
Getting logs off a device -> ios-android-logs.
Offline storage design / SQLite migration -> capacitor-offline-first, sqlite-to-fast-sql.
Splash screen setup -> capacitor-splash-screen; keyboard resize jank -> capacitor-keyboard.
OTA bundle size and delta updates -> capgo-live-updates.

Procedure

Reproduce on a real device in a release-like build. Debug builds log every bridge call and disable optimizations; simulators hide memory and GPU limits.
Classify with the profiling table below: JS main thread busy, bridge payloads large, native thread busy, or waiting on network.
Check the traps list. Most Capacitor-specific regressions are one of these.
Fix one thing, measure again with the same command/trace. Report before/after numbers to the user.

Profiling Quick Reference

| Question | Tool | |---|---| | JS long tasks, layout, paint (iOS) | Safari > Develop > device > app > Timelines. Requires ios.webContentsDebuggingEnabled: true for release builds (debug builds are inspectable by default on iOS 16.4+). | | JS long tasks, memory heap (Android) | chrome://inspect > Performance / Memory. | | Native CPU, allocations, leaks (iOS) | Xcode > Product > Profile > Time Profiler / Allocations / Leaks; field data in Xcode Organizer > Metrics. | | Native CPU/memory (Android) | Android Studio > Profiler. | | Android cold start time | adb shell am force-stop <appId> && adb shell am start -W -n <appId>/.MainActivity (read TotalTime). | | Android frame stats | adb shell dumpsys gfxinfo <appId> (janky frames %). | | Android WebView version | adb shell dumpsys webviewupdate (current provider and version). |

Details and recipes: references/profiling.md.

Capacitor-Specific Traps

Base64 over the bridge. Every plugin call/response is JSON-serialized across the bridge. Multi-MB base64 strings stall both threads and can kill the WebView process.
- Camera (@capacitor/camera 8.1+ API): use takePhoto / chooseFromGallery and display result.webPath; set targetWidth/targetHeight/quality. Avoid reading full-resolution base64 unless uploading. The older getPhoto / pickImages are deprecated.
- Files: pass paths, use Capacitor.convertFileSrc(uri) for <img>/<video> src, Filesystem.readFileInChunks for large reads, @capacitor/file-transfer for uploads/downloads.
WebView process killed under memory pressure. iOS logs WebView process terminated; Capacitor resets the bridge and reloads the page, so in-memory JS state is lost (looks like a restart). Android calls onRenderProcessGone; unless handled, the app is killed. Reduce peak memory (trap 1, large canvases, decoded images), persist critical state, restore on load.
CapacitorHttp for large responses. With plugins.CapacitorHttp.enabled: true, fetch/XMLHttpRequest are routed through native and bodies cross the bridge. Great for CORS/cookies, slower for big downloads or streaming. Use file-transfer for big files.
Preferences as a database. @capacitor/preferences (UserDefaults / SharedPreferences) is for small key/value data. Large JSON blobs are read/written whole and slow startup; use SQLite (for example @capgo/capacitor-fast-sql).
Logging in release. loggingBehavior defaults to debug (logs only in debug builds). If it was set to production, every bridge call is logged in release; set it back to debug or none.
Dev server left in config. server.url pointing to a LAN/live-reload server in a shipped build loads over network and fails offline. Remove before release; in Capacitor 9 use npx cap run <platform> --url for live reload instead of editing config.
Old Android System WebView. Performance and CSS support depend on the updatable WebView, not the OS version. android.minWebViewVersion (default 60) only shows an error/server.errorPath; it does not update anything. Expensive CSS (backdrop-filter, large box-shadow/filter on scrolling content) janks on low-end WebViews.
Listener leaks. addListener returns Promise<PluginListenerHandle>; await it and call remove() on unmount. Re-registering on every route change multiplies native callbacks (appStateChange, keyboardWillShow, geolocation watchPosition - clear with clearWatch).
Startup sequencing. Hide the splash only after first meaningful render (launchAutoHide: false + SplashScreen.hide()), and defer non-critical plugin calls (analytics init, permission checks) until after first paint. Note Capacitor 9 changes Android launchFadeOutDuration default from 200 to 0.
Plugin imports are not the startup cost. Importing @capacitor/* packages creates lightweight proxies; their web implementations are already lazy-loaded via registerPlugin. Lazy-load heavy JS libraries (charts, PDF, maps SDK JS) and routes instead.

Verification

Same device, same build type, before/after numbers: cold start TotalTime (Android) or Instruments App Launch / Organizer launch time (iOS), janky frame %, peak memory in Allocations/Profiler.
Grep the release config: grep -nE '"url"|loggingBehavior|webContentsDebuggingEnabled' capacitor.config.* and confirm no dev server.url.
For trap 1, grep for full-size base64 usage: grep -rnE "resultType: *CameraResultType.Base64|readFile\(|getPhoto\(" src/.
Background the app, open the camera 10 times, return: the page must not reload (iOS) or crash (Android).

Error Handling

| Message / symptom | Meaning | Fix | |---|---|---| | WebView process terminated (Xcode console) | WKWebView content process killed (memory) | Trap 1/2: lower peak memory, persist state. | | Android logcat Render process ... gone / app exits after heavy page | Renderer crashed or killed | Reduce memory; implement handling via a WebViewListener onRenderProcessGone if recovery is required. | | ANR in <appId> (logcat) | Main thread blocked > 5 s, often heavy plugin work on UI thread | Move native work off main thread in custom plugins; profile with Android Studio. | | Message from debugger: Terminated due to memory issue | iOS jetsam killed the whole app | Allocations instrument; reduce image decoding and caches. | | Logcat error about the WebView version being below minWebViewVersion | Device WebView too old | Ask user to update Android System WebView; show server.errorPath page. |

References

Only load when needed:

references/profiling.md: step-by-step traces (Safari Timelines, Chrome Performance, Instruments templates, Android Profiler, adb commands) and how to read them.