Hi,
If there is difficulty in integration, you may keep NvBufferTransform() and try to set VIC at max clocks:
Nvvideoconvert issue, nvvideoconvert in DS4 is better than Ds5? - #3 by DaneLLL
See if it gives performance improvement.
Actually for achieving 4K 25fps, the bottleneck looks to be in JPEG decoding, not VIC.