when I modify the ffmpeg command line to include pixel reformatting, it takes 3 minutes to convert a 60 second hevc video:
ffmpeg -y -v verbose -c:v hevc_cuvid -i ~/bbb_hevc_3840x1920.mp4 -c:v h264_nvenc -pix_fmt yuv420p -vf scale=1280:720 output.mp4
…
frame= 3604 fps= 21 q=30.0 Lsize= 15326kB time=00:01:00.05 bitrate=2090.8kbits/s dup=4 drop=0 speed=0.346x
video:15311kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.101102%
Input file #0 (/home/triveni/bbb_hevc_3840x1920.mp4):
Input stream #0:0 (video): 3600 packets read (59481298 bytes); 3600 frames decoded;
Total: 3600 packets (59481298 bytes) demuxed
Output file #0 (output.mp4):
Output stream #0:0 (video): 3604 frames encoded; 3604 packets muxed (15678243 bytes);
Total: 3604 packets (15678243 bytes) muxed
[h264_nvenc @ 0x2881280] Nvenc unloaded
started at Fri Mar 16 17:20:10 EDT 2018
ended at Fri Mar 16 17:23:04 EDT 2018