buffer: optimize createFromString

PR-URL: nodejs#54324

lib/buffer.js

@@ -442,21 +442,26 @@ function allocate(size) {
 }
 
 function fromStringFast(string, ops) {
-  const length = ops.byteLength(string);
+  let length = string.length * 4; // max utf8 byte length
+
+  if (length >= (Buffer.poolSize >>> 1))
+    length = ops.byteLength(string);
 
   if (length >= (Buffer.poolSize >>> 1))
     return createFromString(string, ops.encodingVal);
 
   if (length > (poolSize - poolOffset))
     createPool();
+
   let b = new FastBuffer(allocPool, poolOffset, length);
   const actual = ops.write(b, string, 0, length);
   if (actual !== length) {
-    // byteLength() may overestimate. That's a rare case, though.
     b = new FastBuffer(allocPool, poolOffset, actual);
   }
+
   poolOffset += actual;
   alignPool();
+
   return b;
 }