No matter what I do, I cannot seem to change the speed of the servo while using this code.
The servo code I am using is from an example that I have adapted. And the original example code allows for the servo to change it's speed. I don't think I have changed anything major to remove this ability, but in my own code, changing the servoInterval does absolutely nothing!
I am stumped 
Here is the code I am borrowing from for reference (edited down to show only the servo stuff)
#include <Servo.h>
const int servoPin = 9; // the pin number for the servo signal
const int servoMinDegrees = 20; // the limits to servo movement
const int servoMaxDegrees = 150;
Servo myservo; // create servo object to control a servo
int servoPosition = 90; // the current angle of the servo - starting at 90.
int servoSlowInterval = 80; // millisecs between servo moves
int servoFastInterval = 10;
int servoInterval = servoSlowInterval; // initial millisecs between servo moves
int servoDegrees = 2; // amount servo moves at each step
unsigned long currentMillis = 0; // stores the value of millis() in each iteration of loop()
unsigned long previousServoMillis = 0; // the time when the servo was last moved
//========
void setup() {
myservo.write(servoPosition); // sets the initial position
myservo.attach(servoPin);
}
//=======
void loop() {
// Notice that none of the action happens in loop() apart from reading millis()
// it just calls the functions that have the action code
currentMillis = millis(); // capture the latest value of millis()
servoSweep();
}
//========
void servoSweep() {
// this is similar to the servo sweep example except that it uses millis() rather than delay()
// nothing happens unless the interval has expired
// the value of currentMillis was set in loop()
if (currentMillis - previousServoMillis >= servoInterval) {
// its time for another move
previousServoMillis += servoInterval;
servoPosition = servoPosition + servoDegrees; // servoDegrees might be negative
if (servoPosition <= servoMinDegrees) {
// when the servo gets to its minimum position change the interval to change the speed
if (servoInterval == servoSlowInterval) {
servoInterval = servoFastInterval;
}
else {
servoInterval = servoSlowInterval;
}
}
if ((servoPosition >= servoMaxDegrees) || (servoPosition <= servoMinDegrees)) {
// if the servo is at either extreme change the sign of the degrees to make it move the other way
servoDegrees = - servoDegrees; // reverse direction
// and update the position to ensure it is within range
servoPosition = servoPosition + servoDegrees;
}
// make the servo move to the next position
myservo.write(servoPosition);
// and record the time when the move happened
}
}
//=====END