diff --git a/src/net/mail/message.go b/src/net/mail/message.go
index 1502b3596252ba..fbf1fca68f5e68 100644
--- a/src/net/mail/message.go
+++ b/src/net/mail/message.go
@@ -832,7 +832,7 @@ func (p *addrParser) consumeComment() (string, bool) {
 	// '(' already consumed.
 	depth := 1
 
-	var comment string
+	var comment strings.Builder
 	for {
 		if p.empty() || depth == 0 {
 			break
@@ -846,12 +846,12 @@ func (p *addrParser) consumeComment() (string, bool) {
 			depth--
 		}
 		if depth > 0 {
-			comment += p.s[:1]
+			comment.WriteByte(p.s[0])
 		}
 		p.s = p.s[1:]
 	}
 
-	return comment, depth == 0
+	return comment.String(), depth == 0
 }
 
 func (p *addrParser) decodeRFC2047Word(s string) (word string, isEncoded bool, err error) {
diff --git a/src/net/mail/message_test.go b/src/net/mail/message_test.go
index dad9c367f3e451..3393b03af54bf2 100644
--- a/src/net/mail/message_test.go
+++ b/src/net/mail/message_test.go
@@ -6,6 +6,7 @@ package mail
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"mime"
 	"reflect"
@@ -1260,3 +1261,21 @@ func TestEmptyAddress(t *testing.T) {
 		t.Errorf(`ParseAddressList("") = %v, %v, want nil, error`, list, err)
 	}
 }
+
+func BenchmarkConsumeComment(b *testing.B) {
+	for _, n := range []int{10, 100, 1000, 10000} {
+		b.Run(fmt.Sprintf("depth-%d", n), func(b *testing.B) {
+			// Build a deeply nested comment: (((...a...)))
+			open := strings.Repeat("(", n)
+			close := strings.Repeat(")", n)
+			// consumeComment expects the leading '(' already consumed,
+			// so we start with one fewer opening paren and the parser
+			// will handle nesting from there.
+			input := open[:n-1] + "a" + close
+			for b.Loop() {
+				p := addrParser{s: input}
+				p.consumeComment()
+			}
+		})
+	}
+}