diff --git a/selfdrive/boardd/boardd_api_impl.pyx b/selfdrive/boardd/boardd_api_impl.pyx
index 3f50fbaab6..0d428a9259 100644
--- a/selfdrive/boardd/boardd_api_impl.pyx
+++ b/selfdrive/boardd/boardd_api_impl.pyx
@@ -14,6 +14,8 @@ cdef extern void can_list_to_can_capnp_cpp(const vector[can_frame] &can_list, st
 
 def can_list_to_can_capnp(can_msgs, msgtype='can', valid=True):
   cdef vector[can_frame] can_list
+  can_list.reserve(len(can_msgs))
+
   cdef can_frame f
   for can_msg in can_msgs:
     f.address = can_msg[0]