Hey there BalconyGorilla , growth rate is regulated by the Vapor Pressure Deficit (VPD) which is the relationship between temperature, humidity and the plant's own temperature. for optimal VPD you should increase your humidity to around 70/75% using a garden bell or some kind of plastic film to cover your seedling. Outside there's not much you can do , but it does pay off to know about this and take measurements. Hope this helps ! 🚀