Basically, time.mktime calls time and localtime, and then overwrites
those results. Removing these unnecessary calls results in a fairly
noticeable speedup, lower double-digit percentile improvements for
applications that do time parsing, for example.

The patch below is for 2.5, but should apply to more recent versions.

diff --git a/Modules/timemodule.c b/Modules/timemodule.c
index be02ec2..dad235a 100644
--- a/Modules/timemodule.c
+++ b/Modules/timemodule.c
@@ -599,8 +599,6 @@ time_mktime(PyObject *self, PyObject *tup)
 {
        struct tm buf;
        time_t tt;
-       tt = time(&tt);
-       buf = *localtime(&tt);
        if (!gettmarg(tup, &buf))
                return NULL;
        tt = mktime(&buf);