record/stream live camera feed with ffmpeg on Raspberry Pi 2B+

[Andrei2]

Hi there! I'm having a bit of a problem right now. I need to record/stream a feed from a USB camera at 1280x720@30fps. I tried the following command:

sudo ffmpeg -f v4l2 -input_format mjpeg -framerate 30 -video_size 1280x720 -i /dev/video0 -pix_fmt yuv420p -r 30 -c:v h264_v4l2m2m -b:v 5M output_26.mp4

It works well for the first 1-2 seconds, then it slows down and drops frames. example If I use -c:v copy it works without a problem, but I need a h264 video.
I have overclocked the Pi at 1000MHz and I expected better performace. I suspect it's ffmpeg's fault, as it works for a second, but then slows down. Is there anything I can change in my command to fix this issue? Maybe buffer for 10-15 seconds before starting recording/streaming?