perf(sse): skip line splitting for single-line data events

sse.go

@@ -315,12 +315,20 @@ func splitSSELines(b []byte) [][]byte {
 // "\r" of any kind ever reaches the wire from here, regardless of what
 // readSSE upstream of this call did or did not already split it on.
 func (s *sseWriter) writeData(b []byte) error {
-	lines := splitSSELines(b)
-	buf := make([]byte, 0, len(b)+len(lines)*len(sseDataPrefix)+1)
-	for _, line := range lines {
+	var buf []byte
+	if !bytes.ContainsAny(b, "\r\n") {
+		buf = make([]byte, 0, len(sseDataPrefix)+len(b)+2)
 		buf = append(buf, sseDataPrefix...)
-		buf = append(buf, line...)
+		buf = append(buf, b...)
 		buf = append(buf, '\n')
+	} else {
+		lines := splitSSELines(b)
+		buf = make([]byte, 0, len(b)+len(lines)*len(sseDataPrefix)+1)
+		for _, line := range lines {
+			buf = append(buf, sseDataPrefix...)
+			buf = append(buf, line...)
+			buf = append(buf, '\n')
+		}
 	}
 	buf = append(buf, '\n')