It's about 4x too fast

[paul35621]

I'm trying a simple example on my Arduino Mega 2560.
Every second an 'a' should be send via the serial. But it happens about 4 times a second.
Same when I try timer 3.

#include "Timer1.h"

void setup ()
{
    // Set up serial connection
    Serial.begin(9600);
    while (!Serial) {}
  
    // Start timer
    startTimer1(1000 * 1000);
}

void loop () {}

ISR(timer1Event)
{
    Serial.write('a');
}