diff --git a/changes/bug33977 b/changes/bug33977
new file mode 100644
index 0000000000..b424a811a2
--- /dev/null
+++ b/changes/bug33977
@@ -0,0 +1,4 @@
+  o Minor bugfix (refactoring):
+    - Lift circuit_build_times_disabled out of circuit_expire_building loop to
+      save CPU time with many circuits open. Fixes bug 33977; bugfix on
+      0.3.5.9.
diff --git a/src/core/or/circuituse.c b/src/core/or/circuituse.c
index 066d5d437a..419ad07f71 100644
--- a/src/core/or/circuituse.c
+++ b/src/core/or/circuituse.c
@@ -548,9 +548,10 @@ circuit_expire_building(void)
              MAX(get_circuit_build_close_time_ms()*2 + 1000,
                  options->SocksTimeout * 1000));
 
+  bool fixed_time = circuit_build_times_disabled(get_options());
+
   SMARTLIST_FOREACH_BEGIN(circuit_get_global_list(), circuit_t *,victim) {
     struct timeval cutoff;
-    bool fixed_time = circuit_build_times_disabled(get_options());
 
     if (!CIRCUIT_IS_ORIGIN(victim) || /* didn't originate here */
         victim->marked_for_close)     /* don't mess with marked circs */