They have quite a moderate growth rate ime, certainly not on a par with an Oscar for example which grow insanely fast. Mine was about the same size as yours when I got her in Sept 2020. She is now a thick 7". Still a bit of growing to do yet, she should max out at around 10".
As with all growth rate questions you will always get the best growth with a suitably sized tank, a good varied diet, and most important of all, keep on top of your water changes.