What can I do when the video and audio is out of sync during editing?

According to your description, we assume it is caused by the lack of the decoding efficiency. Regardless of what editing you made, such as split a video, to show you the prompt result in the preview window, our program has to render it first. But if too many or too big files are loaded and use up all system RAM, since the timeline engine is not efficient enough, so the non-synced problem occurred.

You can pre-render the video to make the editing more smooth, here is the guidance.

https://filmora.wondershare.com/guide/editing-video.html#render