diff --git a/src/log/slog/handler.go b/src/log/slog/handler.go
index 1ca4f9dba3..39b987b812 100644
--- a/src/log/slog/handler.go
+++ b/src/log/slog/handler.go
@@ -525,8 +525,7 @@ func (s *handleState) appendError(err error) {
 func (s *handleState) appendKey(key string) {
 	s.buf.WriteString(s.sep)
 	if s.prefix != nil && len(*s.prefix) > 0 {
-		// TODO: optimize by avoiding allocation.
-		s.appendString(string(*s.prefix) + key)
+		s.appendTwoStrings(string(*s.prefix), key)
 	} else {
 		s.appendString(key)
 	}
@@ -538,6 +537,24 @@ func (s *handleState) appendKey(key string) {
 	s.sep = s.h.attrSep()
 }
 
+// appendTwoStrings implements appendString(prefix + key), but faster.
+func (s *handleState) appendTwoStrings(x, y string) {
+	buf := *s.buf
+	switch {
+	case s.h.json:
+		buf.WriteByte('"')
+		buf = appendEscapedJSONString(buf, x)
+		buf = appendEscapedJSONString(buf, y)
+		buf.WriteByte('"')
+	case !needsQuoting(x) && !needsQuoting(y):
+		buf.WriteString(x)
+		buf.WriteString(y)
+	default:
+		buf = strconv.AppendQuote(buf, x+y)
+	}
+	*s.buf = buf
+}
+
 func (s *handleState) appendString(str string) {
 	if s.h.json {
 		s.buf.WriteByte('"')
diff --git a/src/log/slog/handler_test.go b/src/log/slog/handler_test.go
index d34025f1bb..9f8d518e96 100644
--- a/src/log/slog/handler_test.go
+++ b/src/log/slog/handler_test.go
@@ -10,7 +10,9 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
+	"log/slog/internal/buffer"
 	"os"
 	"path/filepath"
 	"slices"
@@ -529,6 +531,20 @@ func TestJSONAndTextHandlers(t *testing.T) {
 			wantText: "name.first=Perry name.last=Platypus",
 			wantJSON: `{"name":{"first":"Perry","last":"Platypus"}}`,
 		},
+		{
+			name:    "group and key (or both) needs quoting",
+			replace: removeKeys(TimeKey, LevelKey),
+			attrs: []Attr{
+				Group("prefix",
+					String(" needs quoting ", "v"), String("NotNeedsQuoting", "v"),
+				),
+				Group("prefix needs quoting",
+					String(" needs quoting ", "v"), String("NotNeedsQuoting", "v"),
+				),
+			},
+			wantText: `msg=message "prefix. needs quoting "=v prefix.NotNeedsQuoting=v "prefix needs quoting. needs quoting "=v "prefix needs quoting.NotNeedsQuoting"=v`,
+			wantJSON: `{"msg":"message","prefix":{" needs quoting ":"v","NotNeedsQuoting":"v"},"prefix needs quoting":{" needs quoting ":"v","NotNeedsQuoting":"v"}}`,
+		},
 	} {
 		r := NewRecord(testTime, LevelInfo, "message", callerPC(2))
 		line := strconv.Itoa(r.source().Line)
@@ -732,3 +748,31 @@ func TestDiscardHandler(t *testing.T) {
 	l.Info("info", "a", []Attr{Int("i", 1)})
 	l.Info("info", "a", GroupValue(Int("i", 1)))
 }
+
+func BenchmarkAppendKey(b *testing.B) {
+	for _, size := range []int{5, 10, 30, 50, 100} {
+		for _, quoting := range []string{"no_quoting", "pre_quoting", "key_quoting", "both_quoting"} {
+			b.Run(fmt.Sprintf("%s_prefix_size_%d", quoting, size), func(b *testing.B) {
+				var (
+					hs     = NewJSONHandler(io.Discard, nil).newHandleState(buffer.New(), false, "")
+					prefix = bytes.Repeat([]byte("x"), size)
+					key    = "key"
+				)
+
+				if quoting == "pre_quoting" || quoting == "both_quoting" {
+					prefix[0] = '"'
+				}
+				if quoting == "key_quoting" || quoting == "both_quoting" {
+					key = "ke\""
+				}
+
+				hs.prefix = (*buffer.Buffer)(&prefix)
+
+				for b.Loop() {
+					hs.appendKey(key)
+					hs.buf.Reset()
+				}
+			})
+		}
+	}
+}