I also found out what the problem is because I made a diff of your uploaded source code to my source code from the NVIDIA download center.
The download center provides an outdated version of the multimedia API if one activates the Jetson Nano filter on the left. See here:
https://developer.nvidia.com/embedded/downloads#?search=multimedia%20api&tx=$product,jetson_nano
Then the newest version that is shown is
32.2 from 2019/07/18
if one leaves away the Filter, the newest Multimedia API version that has a dedicated Nano download link is:
32.2.3 from 2019/11/19
(but in this version the argus_camera app has the Segmentation Fault again as described in L4T R32.4.3 Breaks argus_camera and causes segmentation fault (SIGSEGV) - #8 by JerryChang)
I think that is clearly a bug on the NVIDIA homepage and NVIDIA should fix this.
Could you confirm what is the newest Multimedia API package that works for the Jetson Nano without segfaulting and with high FPS?