To use your GPU rather than your CPU to convert videos under AMD architecture, you need to use va-api, video acceleration API.

Results:
51 seconds using CPU 100fps 21mb file-size
34 seconds using GPU 130fps 42mb file-size

Requirements:

# VA-API for Radeon HD 2000 and newer require libva-mesa-driver
# VDPAU for Radeon R300 and newer require mesa-vdpau
# libva-vdpau-driver
# you can just install both

# arch manjaro
sudo pacman -S libva-mesa-driver mesa-vdpau libva-vdpau-driver

# debian ubuntu
apt install mesa-va-drivers -y

You may need to reboot in order for the kernel to load with the new updates.

This will install a library a library for

After installing libva vdpau drivers for mesa, you will have a device driver that you can reference in ffmpeg /dev/dri/renderD128

Convert ffmpeg files using your AMD Graphics Card, Laptop Radeon Vega Mobile works too.

Initialize the device

# init device
ffmpeg -init_hw_device vaapi=gpu:/dev/dri/renderD128

Now you can convert with the GPU:

IN_VIDEO=video1.mp4
OUT_VIDEO=video2.mp4
ffmpeg -i "$IN_VIDEO" -vaapi_device /dev/dri/renderD128 -vcodec h264_vaapi -vf format='nv12|vaapi,hwupload' "$OUT_VIDEO"

How to tell if my GPU is being used to convert videos using the gpu?

Run two tests.

# test 1
ffmpeg -i in.mp4 normal.mp4

# test 2
ffmpeg -i in.mp4 -vaapi_device /dev/dri/renderD128 -vcodec h264_vaapi -vf format='nv12|vaapi,hwupload' accelerated.mp4

The GPU accelerated version using AMD Ryzen 5 3500U with Radeon Vega finished before we could even get a decent CPU reading!