Fastest way I found to compute the UTF-8 length of a UTF-16 string.

utf8-length.js

// larger buffer => faster
const BUFFER_SIZE = 1024;

let buffer;

/**
 * @param {string} text
 * @returns {number}
 */
function utf8Length(text) {
  let length = 0;
  let read = 0;
  let enc = new TextEncoder();
  buffer ??= new Uint8Array(BUFFER_SIZE); // TODO: use a WeakRef so this can be gc'd
  while (read < text.length) {
    const p = enc.encodeInto(text.slice(read), buffer);
    read += p.read;
    length += p.written;
  }
  return length;
}