When I make fake data and run your model as written, it samples extremely fast.
I would check the shapes of all your variables to make sure they are what you expect you are. pm.model_to_graphviz is convenient for doing this. If that’s not the problem, your priors might be too narrow. Standardizing the input data (df in your model) often helps as well (see here for a discussion on scaling/centering and priors)