Arduino Uno lag in simple loop

I have a simple Arduino sketch which I have been playing with on my Uno.

The idea is simple, the user turns a potentiate (pin A2), when they pass a certain threshold a LED is turned on (pin 7).

Here is the code:

// Digital pins
const byte ledPin = 7;

// Analog pins
const byte userInputPotPin = A2;

// Potentiometer constants
const int userInputPotMinimumValue = 0;
const int userInputPotMaximumValue = 1023;
const int userInputMinimumValue = 1;
const int userInputMaximumValue = 100;

// Variables
const int threshold = 90;

void setup()
{
    Serial.begin(9600);

    // Digital pins
    pinMode(ledPin, OUTPUT);

    // Analog pins
    pinMode(userInputPotPin, INPUT);
}

void loop()
{
    int rawValue = analogRead(userInputPotPin);
    int mappedValue = map(rawValue, userInputPotMinimumValue, userInputPotMaximumValue, userInputMinimumValue, userInputMaximumValue);

    String messageText;
    messageText += "Raw:   " + String(rawValue) + "\t";
    messageText += "Mapped:" + String(mappedValue);

    if(mappedValue > threshold)
    {
        messageText += "\tThreshold exceeded!";
        digitalWrite(ledPin, HIGH);
    }
    else
    {
        digitalWrite(ledPin, LOW);
    }

    Serial.println(messageText);

    delay(100);
}

The problem is that when it is run the hardware is very unresponsive - I can turn the pot, but I don't see the light change for several minutes. The log messages in the serial monitor also seem to be lagging behind.

I have tried:
* Removing the Serial statements (so there is no serial communication when the sketch is running), and just observing the LED. This makes no difference.
* Increasing the value in the delay up to 1000. This makes no difference in the responsiveness.

However, when I run the sketch on my Mega2560 the problem goes away and the circuit is as responsive as I expect.

Is there something wrong with my Uno board?