iOS UI Rendering Performance

UI rendering performance of an iOS app is measured by observing whether app interactions in user journeys are natural and instant. The UI rendering app performance of an iOS app is the percentage of slow frames rendered during a session. The percentage of slow frames is calculated using the following formula:

Percentage of slow frames rendered: (Frames taking more than 16 ms to render * 100)/Total number of frames rendered during a session.

In the app performance report, go to the UI Rendering Performance section to view the UI rendering performance of the app during the session. In the graph, the axes represent the following:

• X-axis: Depicts the time (in seconds) elapsed since the start of the session. 0 on the X-axis represents the start of the profiling session.
• Y-axis: Depicts the number of frames rendered per second and slow frames rendered per second.

Example

In the following example, you can see that 50.01% of the total rendered frames during the session were slow (frames that took more than 16 ms to render). This is flagged as an issue in the app performance report because it can impact the user experience. The plot shows the number of frames rendered per second and the number of slow frames rendered per second.

To identify a problem frame, you can play the session video. Alongside the video, a bar moves on the graph to help you identify the frame of your interest. To view frames per second or slow frames per second individually, use the legends given on the graph.

How does it impact user experience

Delay in user interaction, hangs, or hitches can cause user experience issues by making the app feel unresponsive and frustrating for users. These issues can lead to poor user engagement and negatively impact user retention, app reviews, and overall app success.

Threshold Guidelines

Percentage of slow frames in a session are 25% or less of the overall frames rendered.

If your app exceeds this thresholds, App Performance Testing flags an issue in the performance report. To know more about the issue, see App performance issues - UI Rendering Performance.

Recommendations

The following are the recommendations to improve UI Rendering Performance:

• Simplify the app’s view hierarchy and reduce the number of subviews to minimize the rendering workload.
• Optimize images and graphics to reduce their memory footprint and improve rendering speed.
• Use auto-layout constraints and avoid hardcoding frames to improve performance and ensure layout correctness.
• Use Core Animation efficiently and avoid performing heavy animation tasks on the main thread.

Related topics

• View the complete list of performance metrics for iOS