Since the index is always non-negative, use faster unsigned division and modulo.

Modules/_collectionsmodule.c

@@ -780,7 +780,9 @@ deque_item(dequeobject *deque, Py_ssize_t i)
         b = deque->rightblock;
     } else {
         i += deque->leftindex;
-        n = i / BLOCKLEN;
+        assert(i >= 0);
+        n = (Py_ssize_t)((unsigned) i / BLOCKLEN);
+        i = (Py_ssize_t)((unsigned) i % BLOCKLEN);
         i %= BLOCKLEN;
         if (index < (Py_SIZE(deque) >> 1)) {
             b = deque->leftblock;