Yolov3 is very slow

IMHO you need to renounce to use YOLOV3 on Jetson nano, is impossible to use.
You need to choose yolov3-tiny that with darknet could reach 17-18 fps at 416x416.

YoloV3 is wonderful but requires to many resources and in my opinion is required a good server with enough GPU (local or cloud).
Cosidering Jetson Nano consumption, it does a good job anyway.