I would wait until after the 3rd week or you see a slight deficiency to start feeding. With a plant this small you really don't want to overfeed it because it will cause damage and stress the plant slowing growth even more. As far as how slow your plant is growing I'm going to guess it is due to the lack of light hours. If you cant get 16hours of sunlight(even indirect sunlight outside is going to be better than either no light or even indirect sunlight indoors(windows filter out quite a bit of the suns light).
I would personally recommend grabbing a cheap basic grow light to keep the plant under more direct light every day. I recommend high-quality leds because they are some of the best lightings out there, but they are expensive to purchase compared to other lighting. Maybe grab a small HPS light or make an array of home led lights for a lower price if you can get them on sale.
If you can spend a little more money Id recommend grabbing at least a 65w quantum board from kingbrite on Alibaba. Ideally I would get a 120w quantum board for a single plant to grow it out, but 65w would still help a lot just to keep it growing more.