diff --git a/include/sta/Graph.hh b/include/sta/Graph.hh
index 1e173361..1ab2b578 100644
--- a/include/sta/Graph.hh
+++ b/include/sta/Graph.hh
@@ -332,6 +332,7 @@ protected:
   // These fields are written by multiple threads, so they
   // cannot share the same word as the following bit fields.
   uint32_t tag_group_index_;
+  uint32_t object_idx_;
   // Each bit corresponds to a different BFS queue.
   std::atomic<uint8_t> bfs_in_queue_; // 8
 
@@ -343,7 +344,6 @@ protected:
   // This flag distinguishes the driver and load vertices.
   bool is_bidirect_drvr_:1;
 
-  unsigned object_idx_:VertexTable::idx_bits; // 7
   bool is_reg_clk_:1;
   bool is_disabled_constraint_:1;
   bool is_gated_clk_enable_:1;
diff --git a/include/sta/Search.hh b/include/sta/Search.hh
index ccd427d9..a6258e33 100644
--- a/include/sta/Search.hh
+++ b/include/sta/Search.hh
@@ -421,6 +421,7 @@ protected:
 		       DcalcAnalysisPt *dcalc_ap_max);
   void deleteTags();
   void deleteTagsPrev();
+  void deleteUnusedTagGroups();
   void seedInvalidArrivals();
   void seedArrivals();
   void findClockVertices(VertexSet &vertices);
diff --git a/search/Search.cc b/search/Search.cc
index bff6b4c8..82664cae 100644
--- a/search/Search.cc
+++ b/search/Search.cc
@@ -429,8 +429,10 @@ Search::deletePaths(Vertex *vertex)
   debugPrint(debug_, "search", 4, "delete paths %s",
              vertex->name(network_));
   TagGroup *tag_group = tagGroup(vertex);
-  if (tag_group)
+  if (tag_group) {
     graph_->deletePaths(vertex);
+    tag_group->decrRefCount();
+  }
 }
 
 ////////////////////////////////////////////////////////////////
@@ -633,6 +635,16 @@ Search::deleteTagsPrev()
   tag_groups_prev_.clear();
 }
 
+void
+Search::deleteUnusedTagGroups()
+{
+  for (TagGroupIndex i = 0; i < tag_group_next_; i++) {
+    TagGroup *group = tag_groups_[i];
+    if (group && group->refCount() == 0)
+      deleteTagGroup(group);
+  }
+}
+
 VertexSeq
 Search::filteredEndpoints()
 {
@@ -1062,6 +1074,7 @@ Search::findArrivals1(Level level)
   Stats stats(debug_, report_);
   int arrival_count = arrival_iter_->visitParallel(level, arrival_visitor_);
   deleteTagsPrev();
+  deleteUnusedTagGroups();
   stats.report("Find arrivals");
   if (arrival_iter_->empty()
       && invalid_arrivals_->empty()) {
@@ -2757,40 +2770,25 @@ Search::setVertexArrivals(Vertex *vertex,
     TagGroup *prev_tag_group = tagGroup(vertex);
     Path *prev_paths = graph_->paths(vertex);
     TagGroup *tag_group = findTagGroup(tag_bldr);
-    size_t path_count = tag_group->pathCount();
-    // Reuse path array if it is the same size.
-    if (prev_tag_group
-	&& path_count == prev_tag_group->pathCount()) {
+    if (tag_group == prev_tag_group) {
       tag_bldr->copyPaths(tag_group, prev_paths);
-      vertex->setTagGroupIndex(tag_group->index());
-      if (tag_group->hasFilterTag()) {
-        LockGuard lock(filtered_arrivals_lock_);
-        filtered_arrivals_->insert(vertex);
-      }
       requiredInvalid(vertex);
     }
     else {
       if (prev_tag_group) {
         graph_->deletePaths(vertex);
+	prev_tag_group->decrRefCount();
         requiredInvalid(vertex);
       }
+      size_t path_count = tag_group->pathCount();
       Path *paths = graph_->makePaths(vertex, path_count);
       tag_bldr->copyPaths(tag_group, paths);
-
       vertex->setTagGroupIndex(tag_group->index());
-      if (tag_group->hasFilterTag()) {
-        LockGuard lock(filtered_arrivals_lock_);
-        filtered_arrivals_->insert(vertex);
-      }
-    }
-    if (tag_group != prev_tag_group) {
-      LockGuard lock(tag_group_lock_);
       tag_group->incrRefCount();
-      if (prev_tag_group) {
-	prev_tag_group->decrRefCount();
-	if (prev_tag_group->refCount() == 0)
-	  deleteTagGroup(prev_tag_group);
-      }
+    }
+    if (tag_group->hasFilterTag()) {
+      LockGuard lock(filtered_arrivals_lock_);
+      filtered_arrivals_->insert(vertex);
     }
   }
 }
diff --git a/search/Search.i b/search/Search.i
index 76338830..416bab51 100644
--- a/search/Search.i
+++ b/search/Search.i
@@ -252,7 +252,7 @@ vertex_worst_slack_path(Vertex *vertex,
 }
 
 int
-tag_group_path_count()
+tag_group_count()
 {
   return Sta::sta()->tagGroupCount();
 }
diff --git a/search/TagGroup.hh b/search/TagGroup.hh
index 74b073ab..5039bd5e 100644
--- a/search/TagGroup.hh
+++ b/search/TagGroup.hh
@@ -24,6 +24,8 @@
 
 #pragma once
 
+#include <atomic>
+
 #include "Vector.hh"
 #include "Map.hh"
 #include "Iterator.hh"
@@ -75,7 +77,7 @@ protected:
   // tag -> path index
   PathIndexMap *path_index_map_;
   size_t hash_;
-  int ref_count_;
+  std::atomic<int> ref_count_;
   unsigned int index_:tag_group_index_bits;
   bool has_clk_tag_:1;
   bool has_genclk_src_tag_:1;