Make sure your time is set on one shot and create a boolean
if speed == 100 and timerSwitch:
timer.start()
timerSwitch = false
I created variables called:
onready var timer = $Timer
var timerSwitch : bool = true
Hello world will still be called multiple times, depending on how much you set your timer. To perform it only once, my code example but without timers, only the boolean.