Question about nvv4l2decoder element

Did not notice the h265 stream can be 10-bit YUV. You are correct the nv3dsink does not support 10-bit YUV and has to convert the frame data to 8-bit YUV for rendering.

Please execute sudo nvpmodel -m 0, sudo jetson_clocks, and apply this:
Nvvideoconvert issue, nvvideoconvert in DS4 is better than Ds5? - #3 by DaneLLL

And run

gst-launch-1.0 -v rtspsrc protocols=tcp location=rtsp:// latency=0 ! rtph265depay ! h265parse ! nvv4l2decoder enable-max-performance=1 ! nvvidconv ! 'video/x-raw(memory:NVMM), format=(string)NV12' ! fpsdisplaysink text-overlay=0 video-sink=nv3dsink sync=0

See if you can achieve target frame rate.