AFAIK, opencv highgui has a drawing thread that could be scheduled depending on if calling waitKey, as @dusty_nv mentioned.
Furthermore, be aware that this drawing thread on CPU is not very efficient on jetson. You may better use a jetson-utils videoOutput (say to “display://0”).
You would keep the RGB to BGR conversion for using most opencv CPU algorithms, and use BGR2RGB before display.
You may have a look to this C++ example (not using CPU).