FFplay should display the video the fact that it isn’t probably has something to do with the errors you are getting
If you can address these then it may help with the OpenCV delay.
Note: OpenCV uses FFmpeg for RTSP streaming by default so you need that to work reliably first.
Are you streaming from an IP camera or rtsp server? Is it on your local network on the same subnet or are you routing accross subnets? Do you have routers/switches in between your computer and the rtsp server which could be causing packet loss?