Hi,
It is possible to see delay if you run OpenCV on Jetson Nano. There is discussion in
[Gstreamer] nvvidconv, BGR as INPUT - #2 by DaneLLL
We would suggest use gstreamer or jetson_multiemdia_api on Jetson Nano. If you have to use OpenCv, please execute sudo nvpmodel -m 0 and sudo jetson_clocks, to run CPU cores at max clocks.