diff --git a/HISTORY.md b/HISTORY.md
index 96e920909..24ce4dc1e 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -29,6 +29,9 @@
 * Add `file_temperature` to `IngestExternalFileArg` such that when ingesting SST files, we are able to indicate the temperature of the this batch of files.
 * If `DB::Close()` failed with a non aborted status, calling `DB::Close()` again will return the original status instead of Status::OK.
 
+### Performance Improvements
+* Improved CPU efficiency of building block-based table (SST) files (#9039 and #9040).
+
 ## 6.25.0 (2021-09-20)
 ### Bug Fixes
 * Allow secondary instance to refresh iterator. Assign read seq after referencing SuperVersion.
diff --git a/table/block_based/block_based_table_builder.cc b/table/block_based/block_based_table_builder.cc
index c94004815..8fb51e17c 100644
--- a/table/block_based/block_based_table_builder.cc
+++ b/table/block_based/block_based_table_builder.cc
@@ -961,8 +961,8 @@ void BlockBasedTableBuilder::Add(const Slice& key, const Slice& value) {
       }
     }
 
+    r->data_block.AddWithLastKey(key, value, r->last_key);
     r->last_key.assign(key.data(), key.size());
-    r->data_block.Add(key, value);
     if (r->state == Rep::State::kBuffered) {
       // Buffered keys will be replayed from data_block_buffers during
       // `Finish()` once compression dictionary has been finalized.
diff --git a/table/block_based/block_builder.cc b/table/block_based/block_builder.cc
index 219504d50..850693454 100644
--- a/table/block_based/block_builder.cc
+++ b/table/block_based/block_builder.cc
@@ -78,6 +78,9 @@ void BlockBuilder::Reset() {
   if (data_block_hash_index_builder_.Valid()) {
     data_block_hash_index_builder_.Reset();
   }
+#ifndef NDEBUG
+  add_with_last_key_called_ = false;
+#endif
 }
 
 void BlockBuilder::SwapAndReset(std::string& buffer) {
@@ -138,33 +141,62 @@ Slice BlockBuilder::Finish() {
 
 void BlockBuilder::Add(const Slice& key, const Slice& value,
                        const Slice* const delta_value) {
+  // Ensure no unsafe mixing of Add and AddWithLastKey
+  assert(!add_with_last_key_called_);
+
+  AddWithLastKeyImpl(key, value, last_key_, delta_value, buffer_.size());
+  if (use_delta_encoding_) {
+    // Update state
+    // We used to just copy the changed data, but it appears to be
+    // faster to just copy the whole thing.
+    last_key_.assign(key.data(), key.size());
+  }
+}
+
+void BlockBuilder::AddWithLastKey(const Slice& key, const Slice& value,
+                                  const Slice& last_key_param,
+                                  const Slice* const delta_value) {
+  // Ensure no unsafe mixing of Add and AddWithLastKey
+  assert(last_key_.empty());
+#ifndef NDEBUG
+  add_with_last_key_called_ = false;
+#endif
+
+  // Here we make sure to use an empty `last_key` on first call after creation
+  // or Reset. This is more convenient for the caller and we can be more
+  // clever inside BlockBuilder. On this hot code path, we want to avoid
+  // conditional jumps like `buffer_.empty() ? ... : ...` so we can use a
+  // fast min operation instead, with an assertion to be sure our logic is
+  // sound.
+  size_t buffer_size = buffer_.size();
+  size_t last_key_size = last_key_param.size();
+  assert(buffer_size == 0 || buffer_size >= last_key_size);
+
+  Slice last_key(last_key_param.data(), std::min(buffer_size, last_key_size));
+
+  AddWithLastKeyImpl(key, value, last_key, delta_value, buffer_size);
+}
+
+inline void BlockBuilder::AddWithLastKeyImpl(const Slice& key,
+                                             const Slice& value,
+                                             const Slice& last_key,
+                                             const Slice* const delta_value,
+                                             size_t buffer_size) {
   assert(!finished_);
   assert(counter_ <= block_restart_interval_);
   assert(!use_value_delta_encoding_ || delta_value);
   size_t shared = 0;  // number of bytes shared with prev key
   if (counter_ >= block_restart_interval_) {
     // Restart compression
-    restarts_.push_back(static_cast<uint32_t>(buffer_.size()));
+    restarts_.push_back(static_cast<uint32_t>(buffer_size));
     estimate_ += sizeof(uint32_t);
     counter_ = 0;
-
-    if (use_delta_encoding_) {
-      // Update state
-      last_key_.assign(key.data(), key.size());
-    }
   } else if (use_delta_encoding_) {
-    Slice last_key_piece(last_key_);
     // See how much sharing to do with previous string
-    shared = key.difference_offset(last_key_piece);
-
-    // Update state
-    // We used to just copy the changed data here, but it appears to be
-    // faster to just copy the whole thing.
-    last_key_.assign(key.data(), key.size());
+    shared = key.difference_offset(last_key);
   }
 
   const size_t non_shared = key.size() - shared;
-  const size_t curr_size = buffer_.size();
 
   if (use_value_delta_encoding_) {
     // Add "<shared><non_shared>" to buffer_
@@ -194,7 +226,7 @@ void BlockBuilder::Add(const Slice& key, const Slice& value,
   }
 
   counter_++;
-  estimate_ += buffer_.size() - curr_size;
+  estimate_ += buffer_.size() - buffer_size;
 }
 
 }  // namespace ROCKSDB_NAMESPACE
diff --git a/table/block_based/block_builder.h b/table/block_based/block_builder.h
index e3fcfc2ec..7702ef117 100644
--- a/table/block_based/block_builder.h
+++ b/table/block_based/block_builder.h
@@ -37,9 +37,24 @@ class BlockBuilder {
 
   // REQUIRES: Finish() has not been called since the last call to Reset().
   // REQUIRES: key is larger than any previously added key
+  // DO NOT mix with AddWithLastKey() between Resets. For efficiency, use
+  // AddWithLastKey() in contexts where previous added key is already known
+  // and delta encoding might be used.
   void Add(const Slice& key, const Slice& value,
            const Slice* const delta_value = nullptr);
 
+  // A faster version of Add() if the previous key is already known for all
+  // Add()s.
+  // REQUIRES: Finish() has not been called since the last call to Reset().
+  // REQUIRES: key is larger than any previously added key
+  // REQUIRES: if AddWithLastKey has been called since last Reset(), last_key
+  // is the key from most recent AddWithLastKey. (For convenience, last_key
+  // is ignored on first call after creation or Reset().)
+  // DO NOT mix with Add() between Resets.
+  void AddWithLastKey(const Slice& key, const Slice& value,
+                      const Slice& last_key,
+                      const Slice* const delta_value = nullptr);
+
   // Finish building the block and return a slice that refers to the
   // block contents.  The returned slice will remain valid for the
   // lifetime of this builder or until Reset() is called.
@@ -60,6 +75,11 @@ class BlockBuilder {
   bool empty() const { return buffer_.empty(); }
 
  private:
+  inline void AddWithLastKeyImpl(const Slice& key, const Slice& value,
+                                 const Slice& last_key,
+                                 const Slice* const delta_value,
+                                 size_t buffer_size);
+
   const int block_restart_interval_;
   // TODO(myabandeh): put it into a separate IndexBlockBuilder
   const bool use_delta_encoding_;
@@ -73,6 +93,9 @@ class BlockBuilder {
   bool finished_;  // Has Finish() been called?
   std::string last_key_;
   DataBlockHashIndexBuilder data_block_hash_index_builder_;
+#ifndef NDEBUG
+  bool add_with_last_key_called_ = false;
+#endif
 };
 
 }  // namespace ROCKSDB_NAMESPACE