diff --git a/pageserver/src/pgdatadir_mapping.rs b/pageserver/src/pgdatadir_mapping.rs
index 15d5609ceb..7b2f71b74e 100644
--- a/pageserver/src/pgdatadir_mapping.rs
+++ b/pageserver/src/pgdatadir_mapping.rs
@@ -205,6 +205,77 @@ impl Timeline {
         Ok(total_blocks)
     }
 
+    pub async fn get_rel_size_if_exists(
+        &self,
+        tag: RelTag,
+        lsn: Lsn,
+        latest: bool,
+        ctx: &RequestContext,
+    ) -> Result<Option<BlockNumber>, PageReconstructError> {
+        if tag.relnode == 0 {
+            return Err(PageReconstructError::Other(
+                RelationError::InvalidRelnode.into(),
+            ));
+        }
+
+        // NOTE there exist helper functions for this but we inline it all
+        //      so we can do the work without acquiring lock more than once
+        //      and searching the hashmap more than once. This is a performance
+        //      critical path.
+        // TODO we could get away with read lock if latest != false. But latest
+        //      is not known at compile time so this work requires some rust type
+        //      acrobatics to do. For now it's fine because we only call this
+        //      with latest = true.
+        let mut rel_size_cache = self.rel_size_cache.write().unwrap();
+
+        match rel_size_cache.entry(tag) {
+            hash_map::Entry::Occupied(entry) => {
+                return Ok(Some(entry.get().1));
+            }
+            hash_map::Entry::Vacant(entry) => {
+                let exists = {
+                    let key = rel_dir_to_key(tag.spcnode, tag.dbnode);
+                    let buf = self.get(key, lsn, ctx).await?;
+                    match RelDirectory::des(&buf).context("deserialization failure") {
+                        Ok(dir) => dir.rels.get(&(tag.relnode, tag.forknum)).is_some(),
+                        Err(e) => return Err(PageReconstructError::from(e)),
+                    }
+                };
+
+                // Return early if not exists
+                if !exists {
+                    if tag.forknum == FSM_FORKNUM || tag.forknum == VISIBILITYMAP_FORKNUM {
+                        // FIXME: Postgres sometimes calls smgrcreate() to create
+                        // FSM, and smgrnblocks() on it immediately afterwards,
+                        // without extending it.  Tolerate that by claiming that
+                        // any non-existent FSM fork has size 0.
+                        return Ok(Some(0));
+                    } else {
+                        return Ok(None);
+                    }
+                }
+
+                // Get size from repo
+                let key = rel_size_to_key(tag);
+                let mut buf = self.get(key, lsn, ctx).await?;
+                let nblocks = buf.get_u32_le();
+
+                if latest {
+                    // Update relation size cache only if "latest" flag is set.
+                    // This flag is set by compute when it is working with most recent version of relation.
+                    // Typically master compute node always set latest=true.
+                    // Please notice, that even if compute node "by mistake" specifies old LSN but set
+                    // latest=true, then it can not cause cache corruption, because with latest=true
+                    // pageserver choose max(request_lsn, last_written_lsn) and so cached value will be
+                    // associated with most recent value of LSN.
+                    entry.insert((lsn, nblocks));
+                }
+
+                Ok(Some(nblocks))
+            }
+        }
+    }
+
     /// Get size of a relation file
     pub async fn get_rel_size(
         &self,
diff --git a/pageserver/src/walingest.rs b/pageserver/src/walingest.rs
index 14a8a956c5..74489961ed 100644
--- a/pageserver/src/walingest.rs
+++ b/pageserver/src/walingest.rs
@@ -1440,27 +1440,21 @@ impl<'a> WalIngest<'a> {
         // record.
         // TODO: would be nice if to be more explicit about it
         let last_lsn = modification.lsn;
-        let old_nblocks = self.timeline.get_rel_size(rel, last_lsn, true, ctx).await?;
-
-        // Create relation if not exists.
-        //
-        // NOTE: We check that old_nblocks == 0 as an optimization. At the time of
-        //       writing, this sped up walingest by 13% on pgbench init. The bottleneck
-        //       here is redundant calls to relation size cache and redundant locking
-        //       in all of these functions. The proper solution is to manually inline
-        //       and lock only once.
-        if old_nblocks == 0
-            && !self
-                .timeline
-                .get_rel_exists(rel, last_lsn, true, ctx)
-                .await?
+        let old_nblocks = match self
+            .timeline
+            .get_rel_size_if_exists(rel, last_lsn, true, ctx)
+            .await?
         {
-            // create it with 0 size initially, the logic below will extend it
-            modification
-                .put_rel_creation(rel, 0, ctx)
-                .await
-                .context("Relation Error")?;
-        }
+            Some(nblocks) => nblocks,
+            None => {
+                // create it with 0 size initially, the logic below will extend it
+                modification
+                    .put_rel_creation(rel, 0, ctx)
+                    .await
+                    .context("Relation Error")?;
+                0
+            }
+        };
 
         if new_nblocks > old_nblocks {
             //info!("extending {} {} to {}", rel, old_nblocks, new_nblocks);