Index: string.c
===================================================================
--- string.c	(revision 56246)
+++ string.c	(working copy)
@@ -8039,13 +8039,19 @@
     const char *const start = s;
 
     if (!s || s >= e) return 0;
+
     /* remove spaces at head */
-    while (s < e) {
-	int n;
-	unsigned int cc = rb_enc_codepoint_len(s, e, &n, enc);
+    if (single_byte_optimizable(str)) {
+        while (s < e && ascii_isspace(*s)) s++;
+    }
+    else {
+	while (s < e) {
+	    int n;
+	    unsigned int cc = rb_enc_codepoint_len(s, e, &n, enc);
 
-	if (!rb_isspace(cc)) break;
-	s += n;
+	    if (!rb_isspace(cc)) break;
+	    s += n;
+	}
     }
     return s - start;
 }