diff --git a/packages/firestore/CHANGELOG.md b/packages/firestore/CHANGELOG.md
index 7281ff0d962..0231cea2cd0 100644
--- a/packages/firestore/CHANGELOG.md
+++ b/packages/firestore/CHANGELOG.md
@@ -1,4 +1,6 @@
 # Unreleased
+- [fixed] Fixed a performance regression introduced by the addition of
+  `Query.limitToLast(n: number)` in Firestore 1.7.0 (Firebase 7.3.0) (#2620).
 - [fixed] Fixed an issue where `CollectionReference.add()` would reject
   custom types when using `withConverter()`. (#2606)
 
diff --git a/packages/firestore/src/util/sorted_set.ts b/packages/firestore/src/util/sorted_set.ts
index dc788079b55..100b760102c 100644
--- a/packages/firestore/src/util/sorted_set.ts
+++ b/packages/firestore/src/util/sorted_set.ts
@@ -135,6 +135,13 @@ export class SortedSet<T> {
 
   unionWith(other: SortedSet<T>): SortedSet<T> {
     let result: SortedSet<T> = this;
+
+    // Make sure `result` always refers to the larger one of the two sets.
+    if (result.size < other.size) {
+      result = other;
+      other = this;
+    }
+
     other.forEach(elem => {
       result = result.add(elem);
     });
diff --git a/packages/testing/src/api/index.ts b/packages/testing/src/api/index.ts
index b4114e2de76..5fd64c41b32 100644
--- a/packages/testing/src/api/index.ts
+++ b/packages/testing/src/api/index.ts
@@ -134,7 +134,7 @@ function initializeApp(
         ({
           getToken: async () => ({ accessToken: accessToken }),
           getUid: () => null,
-          addAuthTokenListener: (listener) => {
+          addAuthTokenListener: listener => {
             // Call listener once immediately with predefined accessToken.
             listener(accessToken);
           },