This is a simple script that uses vaapi hardware acceleration to decode and encode videos.

recode_vaapi.sh

#!/bin/bash

# Usage function, displays valid arguments
usage() {
  echo "Usage: $(basename ${0}) [arguments] inputfile" 1>&2
  echo "  -q  qp, defaults to 18 for near lossless quality, higher value = lower quality." 1>&2
  echo -e "\nExample: $(basename ${0}) -q 22" 1>&2
  exit 1
}

# Default variables
qp=18

# getopts to process the command line arguments
while getopts ":q:p:" opt; do
    case "${opt}" in
        q) qp=${OPTARG};;
        *) usage;;
    esac
done

# shift out the arguments already processed with getopts
shift "$((OPTIND - 1))"
if (( $# == 0 )); then
    printf >&2 'Missing input file\n'
    usage >&2
fi

# set input variable to the first option after the arguments
input="$1"

# Extract filename from input file without the extension
filename=${input%.*}
title=${1##*/}
echo "${filename} (qp${qp}).mkv"

ffmpeg -hide_banner -hwaccel vaapi -hwaccel_device /dev/dri/renderD128 -hwaccel_output_format vaapi -i "${input}" -map 0:v? -map 0:a? -map 0:s? -map 0:d? -map 0:t? -vf 'scale_vaapi=format=p010le' -c:v hevc_vaapi -qp ${qp} -profile:v main10 -max_muxing_queue_size 1024 -c:a copy -c:s copy -y "${filename} (qp${qp}) (recode).mkv"

RESULT=$?
if [ $RESULT -eq 0 ]; then
  mkvmerge --output "${filename} (qp${qp}).mkv" "${filename} (qp${qp}) (recode).mkv" --title "${filename}"
  RESULT=$?
else
  echo "ERROR: ffmpeg failed"
fi

if [ $RESULT -eq 0 ]; then
  rm -rf "${filename} (qp${qp}) (recode).mkv"
  RESULT=$?
fi

Tested on a Linux box running Fedora 32 with ffmpeg 4.2.4 from rpmfusion.

CPU is an Intel 10th Gen G6400.

4K HEVC 10bit HDR to 4K HEVC 10bit HDR = 14fps
1080P H264 to 1080P HEVC 10bit = 55fps

During transcodes CPU usage is ~9%
intel-gpu-top:
Render/3D/0 usage is 80-85%
Video/0 usage is 50-80%
VideoEnhance/0 usage is 6-10%