CH340 max serial speed
CHG340 supports common baud rates: 50, 75, 100, 110, 134.5, 150, 300, 600, 900, 1200, 1800, 2400, 3600, 4800, 9600, 14400, 19200, 28800, 33600, 38400, 56000, 57600, 76800, 115200, 128000, 153600, 230400, 460800, 921600, 1500000, 2000000 baud.

250 000 baud isn't supported by that IC.

The Baud steps is either 200% or 150% of the previous value (There is exceptions).

Tested and working Baud rates.

I used putty as serial monitor since the built in monitor didn't have all baud rates preprogrammed.

SpeedTest.ino

// Serial port Baudrates:
// const int Speed = 115200;
// const int Speed = 128000;
// const int Speed = 153600;
// const int Speed = 230400;
// const int Speed = 460800;
// const int Speed = 921600;
// const int Speed = 1500000;
const int Speed = 2000000;

void setup() {
  // Initialize serial:
  Serial.begin(Speed);

  // Initialize digital pin LED_BUILTIN as an output.
  pinMode(LED_BUILTIN, OUTPUT);
}


void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
  Serial.print("Speed :  ");
  Serial.println(Speed);
}