diff --git a/pageserver/src/layered_repository/inmemory_layer.rs b/pageserver/src/layered_repository/inmemory_layer.rs
index 4d7f7169a0..2e0af289ad 100644
--- a/pageserver/src/layered_repository/inmemory_layer.rs
+++ b/pageserver/src/layered_repository/inmemory_layer.rs
@@ -642,18 +642,11 @@ impl InMemoryLayer {
 
         // Divide all the page versions into old and new
         // at the 'cutoff_lsn' point.
-        // TODO OrderedVec split method
-        let mut before_segsizes = OrderedVec::default();
-        let mut after_segsizes = OrderedVec::default();
-        let mut after_oldest_lsn: Accum<Lsn> = Accum(None);
-        for (lsn, size) in inner.segsizes.iter() {
-            if *lsn > cutoff_lsn {
-                after_segsizes.append(*lsn, *size);
-                after_oldest_lsn.accum(min, *lsn);
-            } else {
-                before_segsizes.append(*lsn, *size);
-            }
-        }
+        let (before_segsizes, after_segsizes) = inner.segsizes.copy_split(&Lsn(cutoff_lsn.0 + 1));
+
+        // The iterator is in Lsn order, so the first element will have the smallest Lsn
+        let mut after_oldest_lsn: Accum<Lsn> =
+            Accum(after_segsizes.iter().next().map(|(lsn, _size)| *lsn));
 
         let mut before_page_versions = PageVersions::default();
         let mut after_page_versions = PageVersions::default();
diff --git a/zenith_utils/src/ordered_vec.rs b/zenith_utils/src/ordered_vec.rs
index 49b44bedce..8d3c46afd0 100644
--- a/zenith_utils/src/ordered_vec.rs
+++ b/zenith_utils/src/ordered_vec.rs
@@ -90,6 +90,22 @@ impl<K: Ord + Copy, V> OrderedVec<K, V> {
     pub fn is_empty(&self) -> bool {
         self.0.is_empty()
     }
+
+    pub fn copy_split(&self, key: &K) -> (Self, Self)
+    where
+        K: Clone,
+        V: Clone,
+    {
+        let idx = match self.0.binary_search_by_key(key, extract_key) {
+            Ok(idx) => idx,
+            Err(idx) => idx,
+        };
+
+        (
+            OrderedVec(Vec::from(&self.0[..idx])),
+            OrderedVec(Vec::from(&self.0[idx..])),
+        )
+    }
 }
 
 impl<K: Ord, V> From<BTreeMap<K, V>> for OrderedVec<K, V> {