Some videos have very low key frame rates, which are difficult to preview in real-time. However the melded video will be smooth.
If the video is encoded using a variable bitrate, it is impossible to accurately seek to exact frames. That causes video to hesitate and skip ahead.
Avoid variable bitrate videos, when possible.
Increase the frame rate setting under Options | Playback Settings to match the video so that interframe seeking is not required during preview playback.
Or decrease the frame rate under Options | Playback Settings to reduce the processing required for playback.
GoldWave Inc.