Why is there huge difference in MixedModels fitting time between Julia 0.3.12 (MixedModels 0.3.22) and Julia1.2 (MixedModels 2.2.0)?

Question

I am running a linear mixed effects model with a dataset consisting of ~500K observations. I have a continuous response (i.e. Y), 24 categorical variables (i.e. X1,..., X24)