Eliminating UI Lag in a High‑End Android App: Overdraw, CPU, and Scroll Optimization

Background

An Android engineer observed noticeable UI jitter when scrolling the home page of a real‑estate app, even on high‑end devices with 6 GB RAM, indicating that the problem was not caused by hardware limitations.

Root Cause Analysis

Three main factors were identified:

CPU saturation : Numerous background services and other apps consume CPU cycles that are needed by SurfaceFlinger to forward draw commands to the GPU.

GPU overload (overdraw) : Excessive draw commands cause frames to exceed the 16 ms budget required for smooth UI, leading to dropped frames.

Memory pressure : Although modern phones usually have sufficient RAM, the app can leak memory or hold large caches, which may cause occasional stalls.

Root‑cause analysis typically consumes about 80 % of the debugging effort, while the actual fix takes the remaining 20 %.

Detecting Overdraw

The built‑in Android developer options Debug GPU Overdraw and GPU Rendering Profile were enabled. The visual overlay showed large overdraw regions on the home page, and the rendering histogram indicated that most frames exceeded the 16 ms budget.

Overdraw Reduction Steps

The following actions were taken to eliminate unnecessary drawing operations:

Remove the activity theme background in the XML style or set it to null in onCreate.

Ensure the root ViewGroup of the activity layout does not set a background color; let child views handle their own backgrounds.

Apply the same rule to the fragment’s root layout.

Inspect each card view on the home page for redundant background attributes and remove them.

Check list items (e.g., the recommended house ListView) for unnecessary background settings.

Avoid calling notifyDataSetChanged, invalidate, or requestLayout unless required, as these force view re‑drawing.

If backgrounds overlap, override onDraw and draw everything onto a cached canvas before flushing to the screen.

CPU‑Bound Issues and TraceView Profiling

After reducing overdraw, occasional lag persisted. The Android TraceView tool was used to profile the main thread. The trace revealed that a custom MyScrollView performed heavy work during scrolling, with the onScroll callback taking about 358 ms on the UI thread.

override fun onScroll(scrollY: Int) {
    LjExposureUtil.statistics(mExposureCardList, mVisibleList)
    mRecommendHouseCard?.childExposure()
}

Refactoring the Scroll Logic

To prevent UI blocking during active scrolling, the analytics call was moved to a method that executes only after scrolling stops.

override fun onScrollStop() {
    LjExposureUtil.statistics(mExposureCardList, mVisibleList)
    mRecommendHouseCard?.childExposure()
}

This change removes the heavy work from the continuous scroll callbacks, resulting in smoother scrolling.

Results and Next Steps

After applying the overdraw clean‑up and scroll refactor, the GPU rendering histogram showed that the majority of frames stayed under the 16 ms threshold. Further performance gains can be achieved by replacing the current home page implementation with a RecyclerView based layout.