you just set a variable to your velocity times delta times your speed variable. Your velocity remains sill the same since you only set it once in your script (velocity = Vector2(1, 0)).
To change the velocity of your bullet you will have to set the velocity accordingly:
func _physics_process(delta):
# no delta needed since your are not changing your velocity over time (constant velocity)
velocity = Vector2(speed, 0)
move_and_slide()
In addition there is no real point in constantly setting your velocity to the same value. This would work fine as well: