How to maximize ffmpeg crop and overlay thousand block in same time?

I try to encrypt a frame of video to random of 16x16 block. so the result will be like artifact video. but exactly it can be decode back. only the creation that know the decode algorithm. but my problem is ffmpeg encode so slow. 3 minutes video, 854x480 (480p) https://www.youtube.com/watch?v=dyRsYk0LyA8. this example result frame that have been filter https://i.ibb.co/0nvLzkK/output-9.jpg. each frame have 1589 block. how to speed up this things? 3 minutes only 24 frame done. the vido have 5000 thousand frame, so for 3 minutes video it takes 10 hours. i dont know why ffmpeg only take my cpu usage 25%.

const { spawn } = require('child_process');
const fs = require('fs');

function shuffle(array) {
    let currentIndex = array.length,  randomIndex;
  
    // While there remain elements to shuffle.
    while (currentIndex != 0) {
  
      // Pick a remaining element.
      randomIndex = Math.floor(Math.random() * currentIndex);
      currentIndex--;
  
      // And swap it with the current element.
      [array[currentIndex], array[randomIndex]] = [
        array[randomIndex], array[currentIndex]];
    }
  
    return array;
  }

function filter(width, height) {
    const sizeBlock = 16;
    let filterCommands = '';
    let totalBlock = 0;
    const widthLengthBlock = Math.floor(width / sizeBlock);
    const heightLengthBlock = Math.floor(height / sizeBlock);
    let info = [];

    for (let i=0; i < widthLengthBlock; i++) {
        for (let j=0; j < heightLengthBlock; j++) {
            const xPos = i*sizeBlock;
            const yPos = j*sizeBlock;
            filterCommands += `[0]crop=${sizeBlock}:${sizeBlock}:${(xPos)}:${(yPos)}[c${totalBlock}];`;

            info.push({
                id: totalBlock,
                x: xPos,
                y: yPos
            });

            totalBlock += 1;
        }   
    }

    info = shuffle(info);

    for (let i=0; i < info.length; i++) {
        if (i == 0) filterCommands += '[0]';
        if (i != 0) filterCommands += `[o${i}]`;

        filterCommands += `[c${i}]overlay=x=${info[i].x}:y=${info[i].y}`;

        if (i != (info.length - 1)) filterCommands += `[o${i+1}];`;     
    }

    return filterCommands;
}

const query = filter(854, 480);

fs.writeFileSync('filter.txt', query);

const task = spawn('ffmpeg', [
    '-i',
    'C:\\Software Development\\ffmpeg\\blackpink.mp4',
    '-filter_complex_script',
    'C:\\Software Development\\project\\filter.txt',
    '-c:v',
    'libx264',
    '-preset',
    'ultrafast',
    '-pix_fmt',
    'yuv420p',
    '-c:a',
    'libopus',
    '-progress',
    '-',
    'output.mp4',
    '-y'
], {
    cwd: 'C:\\Software Development\\ffmpeg'
});

task.stdout.on('data', data => { 
    console.log(data.toString())
})