Well if you want to improve growth you have two factors : sunlight and nutrition. You should first make a quick research what the optimum light cycle(daynight) cycle is for citrus plants. Then you can go ahead and make your own compost to gain top quality nutrition, that you have control over so you know no bad stuff is in there, and that should do wonders for your plant. Although your plants look pretty healthy I'd say.

good luck with you gardening :)