Trying to understand AccelStepper constant speed
So I ended up doing some source code diving for the AccelStepper library to figure out what the problem was and it turns out there was a missing method call in their example code, specifically a call to setMaxSpeed in the setup function:
void setup()
{
    Serial.begin(9600); // set up Serial library at 9600 bps
    Serial.println(&quot;Stepper test!&quot;);

    AFMS.begin(); // create with the default frequency 1.6KHz

    // ! Note that you need to set the max speed, otherwise the stepper motor will
    // ! only step once per interval regardless of what `setSpeed` value you give
    Astepper1.setMaxSpeed(1000);

    Astepper1.moveTo(targetDistance);
    Astepper1.setSpeed(targetSpeed);
}

If you look at the setSpeed method in the library you'll notice a constraint of the speed value:

The constrain method takes in a speed value and then a low and high, which the accelstepper library sets to the private _maxSpeed variable. If the speed is outside of that range it will set the value to the low or high value passed in.

This means, if you don't call setMaxSpeed the library will use the default _maxSpeed value assigned during construction which issss 1.0:

So, to correctly use the constant speed methods, you need to be sure to include a call to setMaxSpeed in your code before calling setSpeed, likely in the setup function.
I also shot a video explanation to share with my co-workers. You can see it here: https://vimeo.com/438272826