diff --git a/server/restserver.go b/server/restserver.go
index 53c0e31..c2e5e3e 100644
--- a/server/restserver.go
+++ b/server/restserver.go
@@ -61,8 +61,7 @@ func PutHandler(s *Server, w http.ResponseWriter, r *http.Request) {
 		json.NewEncoder(w).Encode(resp)
 		return
 	}
-	s.store.Put(namespace, key)
-	count := s.store.Count(namespace, key)
+	count := s.store.Put(namespace, key)
 	resp := CountResponse{Count: count}
 	json.NewEncoder(w).Encode(resp)
 }
diff --git a/store/store.go b/store/store.go
index 95e47d6..4688edf 100644
--- a/store/store.go
+++ b/store/store.go
@@ -43,6 +43,7 @@ func (m *Metrics) ListenAndServe(promHostPort string) error {
 type Store struct {
 	sync.Mutex            // mutext locks namespaces
 	namespaces            *hashmap.Map
+	namespaceSnapshot     []string
 	expireAfterSecs       int64
 	cleanupServiceEnabled bool
 	LastMetrics           *Metrics
@@ -78,8 +79,9 @@ func NewStore(expireAfterSecs int64) *Store {
 	registry.MustRegister(maxNamespacesDenomGauge, namespacesTotalCount, namespacesGarbageCollected, keysRemainingInGCNamespaces, countTotalRemainingInGCNamespaces, gcPauseTime)
 
 	s := &Store{
-		expireAfterSecs: expireAfterSecs,
-		namespaces:      hashmap.New(),
+		expireAfterSecs:   expireAfterSecs,
+		namespaces:        hashmap.New(),
+		lastGCdNamespaces: map[string]bool{},
 		LastMetrics: &Metrics{
 			registry:                          registry,
 			maxNamespacesDenom:                maxNamespacesDenomGauge,
@@ -191,24 +193,27 @@ func (s *Store) runCleanup() []string {
 	for _, key := range keys {
 		keyList = append(keyList, key.(string))
 	}
+
+	s.Lock()
+	s.namespaceSnapshot = append([]string(nil), keyList...)
+	s.Unlock()
 	return keyList
 }
 
-func (s *Store) Put(ns, entryKey string) {
+func (s *Store) Put(ns, entryKey string) int {
 	var subtree *tree.Tree
 	s.Lock()
 	subtreeI, found := s.namespaces.Get(ns)
-	s.Unlock()
 	if !found {
 		subtree = tree.NewTree(s.expireAfterSecs)
-		s.Lock()
 		s.namespaces.Put(ns, subtree)
-		s.Unlock()
+		s.namespaceSnapshot = append(s.namespaceSnapshot, ns)
 	} else {
 		subtree = subtreeI.(*tree.Tree)
 	}
+	s.Unlock()
 
-	subtree.Put(entryKey)
+	return subtree.Put(entryKey)
 }
 
 // Count returns the number of entries at a namespace and key, returning
@@ -227,8 +232,12 @@ func (s *Store) Count(ns, entryKey string) int {
 
 // Namespaces returns the approximate current namespaces list
 func (s *Store) Namespaces() []string {
-	keys := s.runCleanup()
-	return keys
+	s.Lock()
+	defer s.Unlock()
+	if len(s.namespaceSnapshot) == 0 {
+		return []string{}
+	}
+	return append([]string(nil), s.namespaceSnapshot...)
 }
 
 // CountEntries returns the count of all entries for the entire namespace.
diff --git a/store/store_test.go b/store/store_test.go
new file mode 100644
index 0000000..56f8f06
--- /dev/null
+++ b/store/store_test.go
@@ -0,0 +1,20 @@
+package store
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStore_NamespacesUsesSnapshot(t *testing.T) {
+	s := NewStore(60)
+	s.DisableCleanup()
+
+	assert.Equal(t, []string{}, s.Namespaces())
+
+	assert.Equal(t, 1, s.Put("namespace0", "key0"))
+	assert.Equal(t, 2, s.Put("namespace0", "key0"))
+	assert.Equal(t, 1, s.Put("namespace1", "key1"))
+
+	assert.ElementsMatch(t, []string{"namespace0", "namespace1"}, s.Namespaces())
+}
diff --git a/store/tree/tree.go b/store/tree/tree.go
index b00c421..21234d1 100644
--- a/store/tree/tree.go
+++ b/store/tree/tree.go
@@ -10,6 +10,11 @@ import (
 
 type ExpireAtSecs int64
 
+type entry struct {
+	expiresAt []int64
+	liveCount int
+}
+
 // Tree is a a thread-safe data structure for tracking expirable items. It automatically expires old entries and keys.
 // It does not garbage collect. Items are only expired when interacting with the data structure.
 type Tree struct {
@@ -60,20 +65,24 @@ func (n *Tree) Count(entryKey string) int {
 	n.Lock()
 	defer n.Unlock()
 
-	datesSecs := n.getAndCleanupUnsafe(entryKey)
-	if datesSecs == nil {
+	item, found := n.getAndCleanupUnsafe(entryKey)
+	if !found {
+		return 0
+	}
+
+	if item.liveCount == 0 {
+		n.tree.Remove(entryKey)
 		return 0
 	}
 
-	if len(*datesSecs) == 0 {
+	item = removeExpired(item)
+	if item.liveCount == 0 {
 		n.tree.Remove(entryKey)
 		return 0
 	}
 
-	datesSecs = removeExpired(datesSecs)
-	count := len(*datesSecs)
-	n.tree.Put(entryKey, *datesSecs)
-	return count
+	n.tree.Put(entryKey, item)
+	return item.liveCount
 }
 
 // KeyMatch crawls the subtree to return keys starting with the `keyPattern` string.
@@ -110,47 +119,52 @@ func (n *Tree) KeyMatch(keyPattern string) []string {
 	return out
 }
 
-func (n *Tree) Put(entryKey string) {
+func (n *Tree) Put(entryKey string) int {
 	n.Lock()
 	defer n.Unlock()
 
-	datesSecs := n.getAndCleanupUnsafe(entryKey)
-	if datesSecs == nil {
-		datesSecs = &[]int64{}
+	item, found := n.getAndCleanupUnsafe(entryKey)
+	if !found {
+		item = entry{}
 	}
-	datesSecs = removeExpired(datesSecs)
+	item = removeExpired(item)
 	secs := time.Now().Unix()
-	nextDatesSecs := append(*datesSecs, secs+n.defaultExpireAfterSecs)
-	n.tree.Put(entryKey, nextDatesSecs)
+	item.expiresAt = append(item.expiresAt, secs+n.defaultExpireAfterSecs)
+	item.liveCount = len(item.expiresAt)
+	n.tree.Put(entryKey, item)
+	return item.liveCount
 }
 
 // getAndCleanupUnsafe does not lock the mutex, so it can be used inside a lock
-func (n *Tree) getAndCleanupUnsafe(entryKey string) *[]int64 {
+func (n *Tree) getAndCleanupUnsafe(entryKey string) (entry, bool) {
 	val, found := n.tree.Get(entryKey)
 	if !found {
-		return nil
+		return entry{}, false
 	}
-	dates := val.([]int64)
-	if len(dates) == 0 {
+	item := val.(entry)
+	if item.liveCount == 0 {
 		// cleanup empty entry
 		n.tree.Remove(entryKey)
-		return nil
+		return entry{}, false
 	}
-	return &dates // not extra copy
+	return item, true
 }
 
-func removeExpired(datesSecs *[]int64) *[]int64 {
-	if len(*datesSecs) == 0 {
-		return datesSecs
+func removeExpired(item entry) entry {
+	if len(item.expiresAt) == 0 {
+		item.liveCount = 0
+		return item
 	}
 	currentTime := time.Now().Unix()
 	var out []int64
 	// TODO: these are already sorted, so we can discard earlier entries
-	for _, removeAt := range *datesSecs {
+	for _, removeAt := range item.expiresAt {
 		if removeAt > currentTime {
 			// KEEP - not expired
 			out = append(out, removeAt)
 		}
 	}
-	return &out
+	item.expiresAt = out
+	item.liveCount = len(out)
+	return item
 }
diff --git a/store/tree/tree_test.go b/store/tree/tree_test.go
index c3beb59..23fe292 100644
--- a/store/tree/tree_test.go
+++ b/store/tree/tree_test.go
@@ -18,10 +18,10 @@ func TestTree_removeExpired(t *testing.T) {
 		time.Now().Unix() - 1,  // expired
 	}
 
-	result := removeExpired(&entries)
-	assert.Equal(t, 2, len(*result))
-	assert.Equal(t, (*result)[0], keep1)
-	assert.Equal(t, (*result)[1], keep2)
+	result := removeExpired(entry{expiresAt: entries, liveCount: len(entries)})
+	assert.Equal(t, 2, result.liveCount)
+	assert.Equal(t, result.expiresAt[0], keep1)
+	assert.Equal(t, result.expiresAt[1], keep2)
 }
 
 func TestTree_Count(t *testing.T) {
@@ -78,6 +78,13 @@ func TestTree_Count(t *testing.T) {
 		assert.Equal(t, 0, len(keys), "Keys() should expire keys")
 		assert.Equal(t, 0, entryCount, "Keys() should expire entries")
 	})
+	t.Run("put returns live count without needing a second scan", func(t *testing.T) {
+		tr := NewTree(2)
+		assert.Equal(t, 1, tr.Put("willy"))
+		assert.Equal(t, 2, tr.Put("willy"))
+		assert.Equal(t, 1, tr.Put("other"))
+		assert.Equal(t, 2, tr.Count("willy"))
+	})
 }
 
 func TestTree_KeyMatch(t *testing.T) {
@@ -125,7 +132,7 @@ func TestTree_KeyMatch(t *testing.T) {
 	t.Run("it does not return the key when all its values are expired", func(t *testing.T) {
 		tr := NewTree(1)
 		tr.Put("a")
-		tr.tree.Put("a", []string{})
+		tr.tree.Put("a", entry{})
 
 		tr.Put("cdbe")
 		tr.Put("cd:aa")