Vulkan Video is 5x slower than NVENC

I’m using an RTX 3080 Ti with driver 610.43.02 on Arch Linux.

I’ve noticed that encoding using vulkan video is very slow.

ffmpeg -init_hw_device vulkan=vk:0 -hwaccel vulkan -hwaccel_output_format vulkan -f lavfi -i testsrc=s=1920x1080:r=1 -vf format=nv12,hwupload=vulkan,fps=60 -c:v hevc_vulkan -b:v 15M -quality 0 -f null /dev/null

Encoding with vulkan video runs at ~194 fps and uses 100% of the encoder.

ffmpeg -hwaccel_device 0 -hwaccel cuda -f lavfi -i testsrc=s=1920x1080:r=1 -vf format=nv12,hwupload_cuda=0,fps=60 -c:v hevc_nvenc -b:v 15M -preset p1 -f null /dev/null

Encoding with NVENC runs at ~910 fps and uses 100% of the encoder.

Why is vulkan video so slow? Is there any way to fix this?