FastLED TeensyLC weird timing behavior

#include "FastLED.h"

//The way this code should behave is there should be 100 LEDS that are lit green
//  and 152 LEDS that are lit blue. And they "chase" eachother.
//  The animation should repeat every 1 second.

class LEDController {
private:
    static const uint8_t LED_COUNT{252};
    CRGB colors[LED_COUNT];
    uint32_t animationDurationMS{1000};
    //color1 is green, color2 is blue
    const CRGB color1{0,255,0}, color2{0,0,100};
    //There will be 100 green LEDs and 152 blue LEDs
    const uint8_t COLOR_1_LED_COUNT{100};
    void updateColors(uint8_t color1StartPos);
public:
    LEDController();
    void advance(uint32_t millisecondsNow);
    void show();
};

void LEDController::updateColors(uint8_t color1StartPos) {
    //Update the values of the array of colors
    for (uint16_t i=0; i<COLOR_1_LED_COUNT; ++i) {
        colors[(i+color1StartPos)%LED_COUNT] = color1;
    }
    for (uint16_t i=COLOR_1_LED_COUNT; i<LED_COUNT; ++i) {
        colors[(i+color1StartPos)%LED_COUNT] = color2;
    }
}

LEDController::LEDController() {
    const uint8_t DATA_PIN = 0;
    FastLED.addLeds<WS2812B, DATA_PIN, GRB>(colors, LED_COUNT);
}

void LEDController::advance(uint32_t millisecondsNow) {
    //This determines where the start of the green LEDs should be
    double percentComplete = static_cast<double>(millisecondsNow)/animationDurationMS;
    uint8_t startingPos = round(percentComplete*LED_COUNT) % LED_COUNT;
    updateColors(startingPos);
}

void LEDController::show() {
    FastLED.show();
}

void setup() {
    //Set serial baud rate
    const int BAUD_RATE = 2400;
    Serial.begin(BAUD_RATE);

    const unsigned long MILLISECOND_SERIAL_WAIT_DELAY = 5;
    while (!Serial) {
        //Wait for serial to init
        delay(MILLISECOND_SERIAL_WAIT_DELAY);
    }

    //Turn orange onboard-LED on to indicate ready status
    //config cpu led pin as output
    pinMode(13, OUTPUT);
    digitalWrite(13, true);
}


void loop() {
    //It's expected that this function will never exit.
    //If it does, it's probably some sort of abnormal failure.
    //In that case, 'loop' will just start over and
    //  it will seem like the board was power cycled
    //  (global function 'setup' is not called again)

    LEDController ledController;

    while (1) {
        uint32_t now = millis();

        //Let the colors advance to the next state.
        ledController.advance(now);

        //Update LEDs respectively
        ledController.show();
    }
}