Videos are recorder from your browser. Hence, once you click the record button, you will have to wait for it to end before you can review it and eventually save it.
When the recording is happening, the top bar of the app becomes red and will show an approximate countdown of when the recording will end.

If you added some music to your flyer the final video will have the same length of the audio track, otherwise the video will be approximately 3 seconds long.

If you ever encounter any issue related to the animation smoothness, you need to use the desktop version of the app which, with its frame-by-frame recording mode, records buttery smooth videos even in very old computers.

If you are using the desktop app and your computer is struggling also when you are editing your flyer, not only when you are recording, you can change the preview quality of your canvas clicking the related selector above it.