# How do I troubleshoot slow first-frame rendering of remote video when using the Agora Video SDK? (/en/api-reference/faq/quality/optimize_video_rendering) > For AI agents: see the complete documentation index at [llms.txt](/llms.txt). When using the Agora Video SDK for real-time Video Calling, Interactive Live Streaming, or Broadcast Streaming, some developers observe that the remote video takes too long to display after joining a channel. This delay can negatively impact user experience, especially in scenarios like live broadcasts where quick video display is critical. ## Why delays occur [#why-delays-occur] The delay in the first frame rendering is usually due to delays across multiple stages, including: * Setting the remote rendering view too late * Not subscribing to the remote video stream in time after joining the channel ## Solution [#solution] Video SDK provides a mechanism to help developers identify which stage of the video rendering pipeline is time-consuming. The `onVideoRenderingTracingResult` callback returns the duration of each stage. By default, durations are calculated from the time `joinChannel` is called. To customize the start time, call `startMediaRenderingTracing`. ### Step 1: Use `startMediaRenderingTracing` before joining [#step-1-use-startmediarenderingtracing-before-joining] To align rendering metrics with actual user experience, call `startMediaRenderingTracing` immediately when the user triggers a join action (such as tapping **Join** or switching channels). This ensures the collected data reflects real-world latency. The following table explains the fields returned in `tracingInfo` from the `onVideoRenderingTracingResult` callback: | Field | Description | Reference Range | Recommendations | | ----------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | `elapsedTime` | * **Decoding event**: Time from `startMediaRenderingTracing` to `onFirstRemoteVideoDecoded`. * **Rendering event**: Time from `startMediaRenderingTracing` to `onFirstRemoteVideoFrame`. | \< 1000 ms for both | If the difference between decoding and rendering is >200 ms, ensure the remote view is set promptly. If only decoding events are reported, check whether:* The remote view is unset * An external renderer is used | | `start2JoinChannel` | Time from `startMediaRenderingTracing` to `joinChannel`. | \< 10 ms | If too large, ensure `joinChannel` is called promptly after user action. | | `join2JoinSuccess` | Time from `joinChannel` to `onJoinChannelSuccess`. | 50–500 ms | If high, check network conditions. Consider calling `preloadChannel` to speed up joining. | | `joinSuccess2RemoteJoined` | Time from `onJoinChannelSuccess` to `onUserJoined`. | ≤50 ms | High values may occur if:* Host joins after the viewer * Streams are not subscribed by default | | `remoteJoined2SetView` | Time from `onUserJoined` to `setupRemoteVideo`. | 0–20 ms | Set the remote view immediately in `onUserJoined`. For optimal performance, pre-set the view before joining. | | `remoteJoined2UnmuteVideo` | Time from `onUserJoined` to subscribing to the video stream. | 0 ms | If high, check if subscription is delayed. Subscribing during join keeps this value at 0. | | `remoteJoined2PacketReceived` | Time from `onUserJoined` to receiving the first video packet. | 0–50 ms | If high, check:* Whether video stream subscription is delayed * Whether the host publishes the stream promptly | ### Step 2: Use default behavior (without `startMediaRenderingTracing`) [#step-2-use-default-behavior-without-startmediarenderingtracing] If you don't call `startMediaRenderingTracing`, the SDK uses the time `joinChannel` is called as the starting point for measuring rendering performance. With this approach, most fields in the `onVideoRenderingTracingResult` callback have the same meaning as when tracing is started before joining. However, two fields have different interpretations, as shown below: | Field | Description | Reference Range | Recommendations | | ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `elapsedTime` | * **Decoding event**: Time from `joinChannel` to `onFirstRemoteVideoDecoded`. * **Rendering event**: Time from `joinChannel` to `onFirstRemoteVideoFrame`. | \< 1000 ms for both | If the difference between decoding and rendering is >200 ms, check whether the remote view is set on time. If only decoding events are received, possible causes include:* Remote view not set * Use of an external renderer | | `start2JoinChannel` | Time from `startMediaRenderingTracing` to `joinChannel`. | 0 ms | This value is always 0 because no tracing was started before calling `joinChannel`. | ### Step 3: Pre-join the channel, then subscribe on demand [#step-3-pre-join-the-channel-then-subscribe-on-demand] If your app requires extremely low latency for the first frame, pre-join the channel without subscribing to the stream. When the user triggers the join action, call `startMediaRenderingTracing` and subscribe to the remote stream. For implementation details, see [Optimize first-frame rendering](/en/video-calling/best-practices/optimize-frame-rendering). The following table describes each field for this scenario: | Field | Description | Reference Range | Recommendations | | ----------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------ | | `elapsedTime` | * **Decoding event**: Time from `startMediaRenderingTracing` to `onFirstRemoteVideoDecoded`. * **Rendering event**: Time from `startMediaRenderingTracing` to `onFirstRemoteVideoFrame`. | \< 500 ms for both | If the rendering time greatly exceeds decoding time, check whether the remote view was set in time. | | `start2JoinChannel` | Time from `joinChannel` to `startMediaRenderingTracing`. | Negative | Since the channel was joined before tracing started, this value is negative and can be ignored. | | `join2JoinSuccess` | Time from `joinChannel` to `onJoinChannelSuccess`. | 50–500 ms | This value is irrelevant because the user has already joined the channel. | | `joinSuccess2RemoteJoined` | Time from `startMediaRenderingTracing` to `onUserJoined`. | 0 ms | If greater than 0, it may indicate frequent channel switching. This value is not meaningful if `onUserJoined` is triggered after tracing starts. | | `remoteJoined2SetView` | Time from `startMediaRenderingTracing` to `setupRemoteVideo`. | 0 ms | If greater than 0, check whether the view was set promptly. This value may be invalid if the view was set before calling `startMediaRenderingTracing`. | | `remoteJoined2UnmuteVideo` | Time from `startMediaRenderingTracing` to subscribing to the stream. | 0 ms | If this value is large, check for delays between calling `startMediaRenderingTracing` and subscribing to the remote stream. | | `remoteJoined2PacketReceived` | Time from `startMediaRenderingTracing` to receiving the first video packet. | \~100 ms | If high, verify that the remote host is publishing video promptly after joining. | ## API Reference [#api-reference] ### Android [#android] * [`joinChannel`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengine.html#api_irtcengine_joinchannel2) * [`startMediaRenderingTracingEx`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengineex.html#api_irtcengineex_startmediarenderingtracingex) * [`onVideoRenderingTracingResult`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengineeventhandler.html#callback_irtcengineeventhandler_onvideorenderingtracingresult) * [`preloadChannel`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengine.html#api_irtcengine_preloadchannel) ### iOS [#ios] * [`joinChannel`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/joinchannel\(bytoken\:channelid\:uid\:mediaoptions\:joinsuccess:\)) * [`startMediaRenderingTracingEx`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/startmediarenderingtracingex\(_:\)) * [`AgoraVideoRenderingTracingInfo`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agoravideorenderingtracinginfo) * [`preloadChannel`](https://api-ref.agora.io/en/video-sdk/ios/4.x/documentation/agorartckit/agorartcenginekit/preloadchannel\(bytoken\:channelid\:uid:\)) ### macOS [#macos] * [`joinChannel`](https://api-ref.agora.io/en/video-sdk/macos/4.x/documentation/agorartckit/agorartcenginekit/joinchannel\(bytoken\:channelid\:uid\:mediaoptions\:joinsuccess:\)) * [`startMediaRenderingTracingEx`](https://api-ref.agora.io/en/video-sdk/macos/4.x/documentation/agorartckit/agorartcenginekit/startmediarenderingtracingex\(_:\)) * [`AgoraVideoRenderingTracingInfo`](https://api-ref.agora.io/en/video-sdk/macos/4.x/documentation/agorartckit/agoravideorenderingtracinginfo) * [`preloadChannel`](https://api-ref.agora.io/en/video-sdk/macos/4.x/documentation/agorartckit/agorartcenginekit/preloadchannel\(bytoken\:channelid\:uid:\)) ### Windows [#windows] * [`joinChannel`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengine.html#api_irtcengine_joinchannel2) * [`startMediaRenderingTracingEx`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengineex.html#api_irtcengineex_startmediarenderingtracingex) * [`onVideoRenderingTracingResult`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengineeventhandler.html#callback_irtcengineeventhandler_onvideorenderingtracingresult) * [`preloadChannel`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengine.html#api_irtcengine_preloadchannel)