How to do "bl-output" for a single NVMM-buffer(NV12) which was been received by appsink?

It looks expected. For dumping YUV data, you would need to set bl-output=0 to get data in pitch linear.

There is blocklinear to pitchlinear conversion on hardware converter. The resolution is high so please run VIC at mas clock:
Nvvideoconvert issue, nvvideoconvert in DS4 is better than Ds5? - #3 by DaneLLL
And also CPU cores at max clock sine you write data to storage. If it still cannot achieve performance requirement, it is vary likely to hit performance constraint. Suggest you not to write every frame to storage. The optimal performance is to keep data in NVMM buffer from source to sink, without writing to another CPU buffer or storage.