is it consider a slow growth outdoor ?

Your Seedling looks fine to me!!👍 The reason it is growing slower right now is because it is focusing its energy on building up its root zone once it does that it will really start to grow so just be patient. As for when you should start feeding, you shouldn't need to feed her for a another week or 2 weeks. The Seedling itself has enough reserves of nutes in it to keep it healthy. make sure you are not over watering you can do this by feeling the weight of the pot when it is dry and then give it its' watering then feel the weight of it once it has been watered make a mental note of what the Dry and Wet weight was and you will know when its time to water pretty simple method yet effective. Happy Growing!!