diff --git a/src/mbyte.c b/src/mbyte.c
index 4905700..7bc184b 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -3067,6 +3067,9 @@
     int
 utf_fold(int a)
 {
+    if (a < 0x80)
+	/* be fast for ASCII */
+	return a >= 0x41 && a <= 0x5a ? a + 32 : a;
     return utf_convert(a, foldCase, (int)sizeof(foldCase));
 }