esptool is limited to 115200 bps when flashing your firmware to an ESP8266, you can try to change this by following these instructions.
esptool.pyfor your installation. On Debian and Ubuntu, this file is located in
esptool.pyand change the value on the line starting with
ESP_ROM_BAUDfrom 115200 to the maximum of 921600 or your preferred baud rate.