Yeah, red devils and oscars spoil a lot of people if they're their first fish. They grow rapidly in length and put on lots of mass after that. Always growing.
My GT only grew about an inch in 6 ish months after he reached around 6". My firemouth, purchased at about 1", is currently 3.5-3.75" after about 19 months.
My flowerhorn on the other hand, was purchased in June of this year at barely 1". He/she is now 3", getting quite stocky, and has some mean teeth coming in.
That being said, stress from aggression does come into play. Either the GT is just exhibiting the slow growth or may be growing even slower because of the JD's aggressiveness.
