[pipeline] Use conj! as the default pipeline rf

src/toucan2/pipeline.clj

@@ -352,7 +352,10 @@
   "The default reducing function for all query types unless otherwise specified. Returns realized maps (by default, Toucan
   2 instances)."
   [_query-type]
-  ((map realize/realize) conj))
+  (fn
+    ([] (transient []))
+    ([acc] (persistent! acc))
+    ([acc x] (conj! acc (realize/realize x)))))
 
 (defn ^:no-doc first-result-xform-fn
   "Return a transducer that transforms a reducing function `rf` so it always takes at most one value and returns the first