first frame freezing for several seconds when streaming with VP8 in real-time

[LazarenkoA]

Hello, earlier I created a question regarding the delay in live streaming:
#2903

In that example, I was capturing video in H264, but I decided to try VP8 following this example: https://github.com/baiyufei/pion-h264-repro, and to my surprise, the delay was gone, and the video streaming indeed became real-time. However, for some reason, at the start of the stream, the first frame appears and freezes for a few seconds, then everything starts working as it should. Why might the first frame be freezing? According to the backend logs, data is being sent.
I’m using the following ffmpeg command for video capture with these parameters:

-loglevel error -f gdigrab -framerate 5 -i desktop -c:v libvpx -pix_fmt yuv420p -vf pad='ceil(iw/2)*2:ceil(ih/2)*2' -threads 1 -b:v 500K -deadline realtime -bf 0 -f ivf pipe:1

[LazarenkoA]

I changed -threads 1 to -threads 0 and the delay passed