diff --git a/plugins/outputs/loki/stream.go b/plugins/outputs/loki/stream.go
index 5af31f94e2744..5f1687ef2fb3b 100644
--- a/plugins/outputs/loki/stream.go
+++ b/plugins/outputs/loki/stream.go
@@ -59,7 +59,7 @@ func uniqKeyFromTagList(ts []*telegraf.Tag) (k string) {
 func newStream(ts []*telegraf.Tag) *Stream {
 	s := &Stream{
 		Logs:   make([]Log, 0),
-		Labels: make(map[string]string),
+		Labels: make(map[string]string, len(ts)),
 	}
 
 	for _, t := range ts {
diff --git a/plugins/outputs/mongodb/mongodb.go b/plugins/outputs/mongodb/mongodb.go
index 5847e0d5eb5a0..60dc930bfca7d 100644
--- a/plugins/outputs/mongodb/mongodb.go
+++ b/plugins/outputs/mongodb/mongodb.go
@@ -26,11 +26,11 @@ import (
 var sampleConfig string
 
 func (s *MongoDB) getCollections(ctx context.Context) error {
-	s.collections = make(map[string]bson.M)
 	collections, err := s.client.Database(s.MetricDatabase).ListCollections(ctx, bson.M{})
 	if err != nil {
 		return fmt.Errorf("unable to execute ListCollections: %w", err)
 	}
+	s.collections = make(map[string]bson.M, collections.RemainingBatchLength())
 	for collections.Next(ctx) {
 		var collection bson.M
 		if err = collections.Decode(&collection); err != nil {
@@ -229,5 +229,9 @@ func (s *MongoDB) Write(metrics []telegraf.Metric) error {
 }
 
 func init() {
-	outputs.Add("mongodb", func() telegraf.Output { return &MongoDB{} })
+	outputs.Add("mongodb", func() telegraf.Output {
+		return &MongoDB{
+			collections: make(map[string]bson.M),
+		}
+	})
 }