diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 59b6fd166c..3c40eb9cef 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -273,6 +273,9 @@ type decodeState struct {
 	savedError            error
 	useNumber             bool
 	disallowUnknownFields bool
+	// safeUnquote is the number of current string literal bytes that don't
+	// need to be unquoted. When negative, no bytes need unquoting.
+	safeUnquote int
 }
 
 // readIndex returns the position of the last byte read.
@@ -374,13 +377,27 @@ func (d *decodeState) rescanLiteral() {
 Switch:
 	switch data[i-1] {
 	case '"': // string
+		// safeUnquote is initialized at -1, which means that all bytes
+		// checked so far can be unquoted at a later time with no work
+		// at all. When reaching the closing '"', if safeUnquote is
+		// still -1, all bytes can be unquoted with no work. Otherwise,
+		// only those bytes up until the first '\\' or non-ascii rune
+		// can be safely unquoted.
+		safeUnquote := -1
 		for ; i < len(data); i++ {
-			switch data[i] {
-			case '\\':
+			if c := data[i]; c == '\\' {
+				if safeUnquote < 0 { // first unsafe byte
+					safeUnquote = int(i - d.off)
+				}
 				i++ // escaped char
-			case '"':
+			} else if c == '"' {
+				d.safeUnquote = safeUnquote
 				i++ // tokenize the closing quote too
 				break Switch
+			} else if c >= utf8.RuneSelf {
+				if safeUnquote < 0 { // first unsafe byte
+					safeUnquote = int(i - d.off)
+				}
 			}
 		}
 	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-': // number
@@ -725,7 +742,7 @@ func (d *decodeState) object(v reflect.Value) error {
 		start := d.readIndex()
 		d.rescanLiteral()
 		item := d.data[start:d.readIndex()]
-		key, ok := unquoteBytes(item)
+		key, ok := d.unquoteBytes(item)
 		if !ok {
 			panic(phasePanicMsg)
 		}
@@ -922,7 +939,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 			d.saveError(&UnmarshalTypeError{Value: val, Type: v.Type(), Offset: int64(d.readIndex())})
 			return nil
 		}
-		s, ok := unquoteBytes(item)
+		s, ok := d.unquoteBytes(item)
 		if !ok {
 			if fromQuoted {
 				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
@@ -973,7 +990,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		}
 
 	case '"': // string
-		s, ok := unquoteBytes(item)
+		s, ok := d.unquoteBytes(item)
 		if !ok {
 			if fromQuoted {
 				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
@@ -1131,7 +1148,7 @@ func (d *decodeState) objectInterface() map[string]interface{} {
 		start := d.readIndex()
 		d.rescanLiteral()
 		item := d.data[start:d.readIndex()]
-		key, ok := unquote(item)
+		key, ok := d.unquote(item)
 		if !ok {
 			panic(phasePanicMsg)
 		}
@@ -1180,7 +1197,7 @@ func (d *decodeState) literalInterface() interface{} {
 		return c == 't'
 
 	case '"': // string
-		s, ok := unquote(item)
+		s, ok := d.unquote(item)
 		if !ok {
 			panic(phasePanicMsg)
 		}
@@ -1223,38 +1240,21 @@ func getu4(s []byte) rune {
 
 // unquote converts a quoted JSON string literal s into an actual string t.
 // The rules are different than for Go, so cannot use strconv.Unquote.
-func unquote(s []byte) (t string, ok bool) {
-	s, ok = unquoteBytes(s)
+func (d *decodeState) unquote(s []byte) (t string, ok bool) {
+	s, ok = d.unquoteBytes(s)
 	t = string(s)
 	return
 }
 
-func unquoteBytes(s []byte) (t []byte, ok bool) {
-	if len(s) < 2 || s[0] != '"' || s[len(s)-1] != '"' {
-		return
-	}
+func (d *decodeState) unquoteBytes(s []byte) (t []byte, ok bool) {
+	r := d.safeUnquote
+	// The bytes have been scanned, so we know that the first and last bytes
+	// are double quotes.
 	s = s[1 : len(s)-1]
 
-	// Check for unusual characters. If there are none,
-	// then no unquoting is needed, so return a slice of the
-	// original bytes.
-	r := 0
-	for r < len(s) {
-		c := s[r]
-		if c == '\\' || c == '"' || c < ' ' {
-			break
-		}
-		if c < utf8.RuneSelf {
-			r++
-			continue
-		}
-		rr, size := utf8.DecodeRune(s[r:])
-		if rr == utf8.RuneError && size == 1 {
-			break
-		}
-		r += size
-	}
-	if r == len(s) {
+	// If there are no unusual characters, no unquoting is needed, so return
+	// a slice of the original bytes.
+	if r == -1 {
 		return s, true
 	}