可以尝试以下两种方法来解决该问题：

1. 使用SPP传输方式：在RN4678中可以使用串行端口协议（SPP）替代RFCOMM进行数据传输，该方式在Android平台下传输速率更快。以下是在Android Studio中使用Java代码连接和发送数据的示例：

private final UUID SPP_UUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB"); //RN4678的SPP服务UUID
private final String MAC_ADDRESS = "xx:xx:xx:xx:xx:xx"; //RN4678的MAC地址

//创建BluetoothSocket实例并连接
BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
BluetoothDevice device = bluetoothAdapter.getRemoteDevice(MAC_ADDRESS);
BluetoothSocket socket = device.createInsecureRfcommSocketToServiceRecord(SPP_UUID);
socket.connect();

//发送数据
OutputStream outputStream = socket.getOutputStream();
String message = "Hello RN4678";
byte[] bytes = message.getBytes(Charset.forName("UTF-8"));
outputStream.write(bytes);

//关闭连接
socket.close();

2. 调整传输参数：RN4678的BLE模块默认的RX间隔和RX窗口较小，可能导致在RFCOMM传输数据时速率较慢。可以通过修改RN4678的传输参数来提高传输速率。以下是在Android Studio中使用AT命令来修改RN4678传输参数的示例：

private final String MAC_ADDRESS = "xx:xx:xx:xx:xx:xx"; //RN4678的MAC地址

//连接设备并发送AT命令
BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
BluetoothDevice device = bluetoothAdapter.getRemoteDevice(MAC_ADDRESS);
BluetoothSocket socket = device.createRfcommSocketToServiceRecord(SerialPortServiceClass_UUID);
socket.connect();

OutputStream outputStream = socket.getOutputStream();
InputStream inputStream = socket.getInputStream();

outputStream.write("AT+CONNL\r\n".getBytes());