Wow it looks like natural light treated you well but if you want to speed up or increase growth buy some cfls to add.

Buy like 4 23 watt cfls and that plant will be worthy of envy.

Cfls are cheap it will prob run you 7$ and 15$ more if you have to buy the holders for the bulbs.

Word of experience though cfls get pretty hot so keep 5 inches away instead of 2 inches away.