The Curv Doorbell depends on a good network connection to run smoothly. The performance depends on your WiFi signal & Internet speed.
The minimum upload speed for 720p video is 512Kb & for 1080p video its 1024Kb for. If the network upload rate is less than minimum rate, the video will be out of sync.
If so, please check with your network provider to see if your broadband speed can be adjusted.
In addition to the above, if there are too many devices connected to the WiFi router, or the device is far away from the WiFi router it may also cause the out of sync video.