Experiment for Latent Gaussian Models

Covariance matrix can be deterministic from \phi, and most often is.

Better to have first working simulation example with speedup, and where you can show the performance with respect to increasing correlation. You could make a plot like in N_eff BDA3 vs. Stan - #21 by avehtari , just use that same multivariate normal as latent function, e.g., for log-intensity of Poisson.

You can generate Poisson distributed data from your model, and you can check how the speed changes when you vary the parameters of data generating model.