important! I am using my ffmpeg compilation from master, so this should work for ffmpeg versions => 6. The current fastest way to create a screencast for videos of any length. A 2.5-hour video in 1080p quality is processed in about 18 seconds. Please note, I use -hwaccel auto which speeds up decoding.

ffmpeg_thumbnails.sh

#!/bin/bash

if [ -z "$1" ]; then
    echo "usage: ffmpeg_thumbnails.sh PATH_TO_VIDEOFILE"
    exit
fi
MOVIE=$1
MOVIE_NAME=$(basename "$MOVIE")
OUT_FILENAME="${MOVIE_NAME%.*}_preview.jpeg"
OUT_DIR=$(pwd)
OUT_FILEPATH="$OUT_DIR/$OUT_FILENAME"
mkdir -p $OUT_DIR
echo $OUT_FILEPATH

WIDTH=720
COLS=8
ROWS=8

TOTAL_DURATION=$(ffprobe -v panic -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "$MOVIE" 2>/dev/null)
TOTAL_IMAGES=$(bc <<<"scale=20;$COLS*$ROWS")
PERIOD_EVERY=$(bc <<<"scale=20;$TOTAL_DURATION/$TOTAL_IMAGES")
vf="fps=1/$PERIOD_EVERY,scale=${WIDTH}:-1,tile=${COLS}x${ROWS}:padding=10:margin=5:nb_frames=0"
ffmpeg -loglevel warning -hwaccel auto -skip_frame nokey -i "$MOVIE" -q:v 1 -frames:v 1 -vf "${vf}" -update 1 -an "$OUT_FILEPATH"