diff -r 5ae4ff03b35f Objects/stringlib/join.h
--- a/Objects/stringlib/join.h	Thu Oct 18 22:18:42 2012 +0100
+++ b/Objects/stringlib/join.h	Fri Oct 19 14:05:51 2012 +0300
@@ -94,6 +94,19 @@
 
     /* Catenate everything. */
     p = STRINGLIB_STR(res);
+    if (!seplen || (seplen == 1 && nbufs > 2 && nbufs > (sz >> 5))) {
+        /* fast path */
+        if (seplen)
+            memset(p + buffers[0].len, sepstr[0],
+                   sz - buffers[0].len - buffers[nbufs - 1].len);
+        for (i = 0; i < nbufs; i++) {
+            Py_ssize_t n = buffers[i].len;
+            char *q = buffers[i].buf;
+            Py_MEMCPY(p, q, n);
+            p += n;
+        }
+        goto done;
+    }
     for (i = 0; i < nbufs; i++) {
         Py_ssize_t n;
         char *q;