Inconsistent fps while recording with ffmpeg

Please refer to steps in
Jetson Nano FAQ
Q: I have a USB camera. How can I launch it on Jetson Nano?

See if you can launch it in gstreamer pipeline. If it can be launched, please try to replace nvoverlaysink with

... ! fpsdisplaysink text-overlay=0 video-sink=nvoverlaysink sync=0 -v

The fps information will be printed out with the plugin.