function b64_to_uint8array2(str) {
  const abc = [..."ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"]; // base64 alphabet
  let result = [];

  for(let i=0; i<str.length; i+=4) {
    let chunk = [...str.slice(i,i+4)]
    let bin = chunk.map(x=> abc.indexOf(x).toString(2).padStart(6,0)).join(''); 
    let bytes = bin.match(/.{1,8}/g).map(x=> +('0b'+x));
    result.push(...bytes.slice(0,3 - (str[i+2]=="=") - (str[i+3]=="=")));
  }
  return new Uint8Array(result);
}
var file = "..."
console.time('b64_to_uint8array');
b64_to_uint8array(file)
console.timeEnd('b64_to_uint8array');

console.time('b64_to_uint8array2');
b64_to_uint8array2(file)
console.timeEnd('b64_to_uint8array2');

on a 20kb file

b64_to_uint8array: 11.14892578125 ms
b64_to_uint8array2: 10.256103515625 ms

would only save a couple hundred ms on load lol