diff --git a/lib/src/local_working_copy.rs b/lib/src/local_working_copy.rs
index d67bcbb05..d45338f20 100644
--- a/lib/src/local_working_copy.rs
+++ b/lib/src/local_working_copy.rs
@@ -149,13 +149,17 @@ impl FileStatesMap {
         FileStatesMap { data: Vec::new() }
     }
 
-    // TODO: skip sorting if the data is known to be sorted?
-    fn from_proto_unsorted(mut data: Vec<crate::protos::working_copy::FileStateEntry>) -> Self {
-        data.sort_unstable_by(|entry1, entry2| {
-            let path1 = RepoPath::from_internal_string(&entry1.path);
-            let path2 = RepoPath::from_internal_string(&entry2.path);
-            path1.cmp(path2)
-        });
+    fn from_proto(
+        mut data: Vec<crate::protos::working_copy::FileStateEntry>,
+        is_sorted: bool,
+    ) -> Self {
+        if !is_sorted {
+            data.sort_unstable_by(|entry1, entry2| {
+                let path1 = RepoPath::from_internal_string(&entry1.path);
+                let path2 = RepoPath::from_internal_string(&entry2.path);
+                path1.cmp(path2)
+            });
+        }
         debug_assert!(is_file_state_entries_proto_unique_and_sorted(&data));
         FileStatesMap { data }
     }
@@ -610,7 +614,8 @@ impl TreeState {
                 .collect();
             self.tree_id = MergedTreeId::Merge(tree_ids_builder.build());
         }
-        self.file_states = FileStatesMap::from_proto_unsorted(proto.file_states);
+        self.file_states =
+            FileStatesMap::from_proto(proto.file_states, proto.is_file_states_sorted);
         self.sparse_patterns = sparse_patterns_from_proto(proto.sparse_patterns.as_ref());
         self.watchman_clock = proto.watchman_clock;
         Ok(())
@@ -630,6 +635,8 @@ impl TreeState {
         }
 
         proto.file_states = self.file_states.data.clone();
+        // `FileStatesMap` is guaranteed to be sorted.
+        proto.is_file_states_sorted = true;
         let mut sparse_patterns = crate::protos::working_copy::SparsePatterns::default();
         for path in &self.sparse_patterns {
             sparse_patterns
@@ -1899,7 +1906,7 @@ mod tests {
             new_proto_entry("b/e", 3),
             new_proto_entry("bc", 5),
         ];
-        let mut file_states = FileStatesMap::from_proto_unsorted(data);
+        let mut file_states = FileStatesMap::from_proto(data, false);
 
         let changed_file_states = vec![
             new_owned_entry("aa", 10),    // change
diff --git a/lib/src/protos/working_copy.proto b/lib/src/protos/working_copy.proto
index 536ca32e6..634c25da2 100644
--- a/lib/src/protos/working_copy.proto
+++ b/lib/src/protos/working_copy.proto
@@ -47,6 +47,7 @@ message TreeState {
   // single (positive) value
   repeated bytes tree_ids = 5;
   repeated FileStateEntry file_states = 2;
+  bool is_file_states_sorted = 6;
   SparsePatterns sparse_patterns = 3;
   WatchmanClock watchman_clock = 4;
 }
diff --git a/lib/src/protos/working_copy.rs b/lib/src/protos/working_copy.rs
index 4291e9549..90c1bfe67 100644
--- a/lib/src/protos/working_copy.rs
+++ b/lib/src/protos/working_copy.rs
@@ -38,6 +38,8 @@ pub struct TreeState {
     pub tree_ids: ::prost::alloc::vec::Vec<::prost::alloc::vec::Vec<u8>>,
     #[prost(message, repeated, tag = "2")]
     pub file_states: ::prost::alloc::vec::Vec<FileStateEntry>,
+    #[prost(bool, tag = "6")]
+    pub is_file_states_sorted: bool,
     #[prost(message, optional, tag = "3")]
     pub sparse_patterns: ::core::option::Option<SparsePatterns>,
     #[prost(message, optional, tag = "4")]