diff --git a/app/controllers/projects/branches_controller.rb b/app/controllers/projects/branches_controller.rb
index 965cece600e..176679f0849 100644
--- a/app/controllers/projects/branches_controller.rb
+++ b/app/controllers/projects/branches_controller.rb
@@ -21,17 +21,13 @@ class Projects::BranchesController < Projects::ApplicationController
         fetch_branches_by_mode
 
         @refs_pipelines = @project.pipelines.latest_successful_for_refs(@branches.map(&:name))
-        @merged_branch_names =
-          repository.merged_branch_names(@branches.map(&:name))
-        # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37429
-        Gitlab::GitalyClient.allow_n_plus_1_calls do
-          @max_commits = @branches.reduce(0) do |memo, branch|
-            diverging_commit_counts = repository.diverging_commit_counts(branch)
-            [memo, diverging_commit_counts[:behind], diverging_commit_counts[:ahead]].max
-          end
-
-          render
+        @merged_branch_names = repository.merged_branch_names(@branches.map(&:name))
+        @max_commits = @branches.reduce(0) do |memo, branch|
+          diverging_commit_counts = repository.diverging_commit_counts(branch)
+          [memo, diverging_commit_counts[:behind], diverging_commit_counts[:ahead]].max
         end
+
+        render
       end
       format.json do
         branches = BranchesFinder.new(@repository, params).execute
diff --git a/app/models/repository.rb b/app/models/repository.rb
index 42f1ac43e29..904bfe06ce6 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -253,13 +253,13 @@ class Repository
   end
 
   def diverging_commit_counts(branch)
-    root_ref_hash = raw_repository.commit(root_ref).id
+    @root_ref_hash ||= raw_repository.commit(root_ref).id
     cache.fetch(:"diverging_commit_counts_#{branch.name}") do
       # Rugged seems to throw a `ReferenceError` when given branch_names rather
       # than SHA-1 hashes
       number_commits_behind, number_commits_ahead =
         raw_repository.count_commits_between(
-          root_ref_hash,
+          @root_ref_hash,
           branch.dereferenced_target.sha,
           left_right: true,
           max_count: MAX_DIVERGING_COUNT)
diff --git a/changelogs/unreleased/44657-reuse-root_ref_hash-on-branches.yml b/changelogs/unreleased/44657-reuse-root_ref_hash-on-branches.yml
new file mode 100644
index 00000000000..4f21aadd86b
--- /dev/null
+++ b/changelogs/unreleased/44657-reuse-root_ref_hash-on-branches.yml
@@ -0,0 +1,5 @@
+---
+title: Reuse root_ref_hash for performance on Branches
+merge_request: 17998
+author: Takuya Noguchi
+type: performance