How to speed up sampling in STAN when fitting a spatial survival model on a large data set?

Dear all,
I’m trying to fit a spatial survival model using RSTAN for large data (N=35000). But, I found sampling is very time-consuming (takes ~ 10 days even for 2000 iteration/500 warmup).
Could anyone help me to solve this problem, please?
Any tips and comments are welcome. Thanks in advance!

My stan code is as below (N=35000, n_grid=350, province=31, num=12).
In my codes, x[j]'s (j=1,2, …, n_grid) are white noise and It is used only in the calculation of a and there is no need to estimate it.

fit < - stan(“final model.stan”,data = da,iter=iter, warmup=warmup, cores =4,chains = 2, thin = 1, init = “random”,control=list(max_treedepth=15))
data {
  int <lower=0> province;
  int <lower=0> n_grid;
  int <lower=0> N;
  int <lower=0> num;
  int <lower=0> number;
  vector <lower=0> [province] p;
  row_vector[n_grid] kernel[province];
  matrix [N,number] respon;
  vector [num] beta;
  real <lower=0> sigma0;
  real <lower=0> sigma;
  vector [n_grid] x;
transformed parameters{
  real alpha = 1 / sigma;
  vector [N] lambdaa;
  vector [n_grid] exp_x;
  vector[province] a; // `a` saved to output 
    exp_x = exp(x);
     lambdaa = exp ((-1 * respon[ ,6:17] * beta) / sigma);
           { // `z`  not part of output
    vector[province] z;
      vector[province] landa;
for (k in 1 : province) {
     landa[k] = kernel[k] * exp_x * p[k];
     a = landa / sum(landa); // assign `a`
     target += normal_lpdf(x|0,5);
    target += normal_lpdf(beta | 0, sigma0);
    target += inv_gamma_lpdf(sigma0| 0.001, 0.001);
    target += normal_lpdf(log (alpha) | 0, 5);
for (i in 1:N) {
   vector [province] f_i;
  vector [province] s_i;
for (k in 1:province){
    f_i[k]= a[k]*((lambdaa [i]*alpha*respon[i,4]^(alpha-1))/((1+(lambdaa [i]*respon[i,4]^alpha))^2));
    s_i[k]= a[k]*(1- ((lambdaa [i]*respon[i,4]^alpha)/ (1+(lambdaa [i]*respon[i,4]^alpha))));
    target += (respon[i,5] *  log (f_i)) + ((1 - respon[i,5])* log(s_i));

EDIT: @maxbiostat edited this post for syntax highlighting.

Just a quick clarification about your model first.

You’ve specified the parameter sigma with an inv_gamma(0.001, 0.001) prior, which is highly diffuse (which we don’t really recommend with Stan, as an aside):

target += inv_gamma_lpdf(sigma0| 0.001, 0.001);

Then you’ve specified the transformed parameter alpha as the inverse of sigma:

  real alpha = 1 / sigma;

Then you’ve assigned a prior to the log of alpha:

  target += normal_lpdf(log(alpha) | 0, 5);

If you assign a prior to a transformation of a parameter, then you need to add a jacobian correction to the log-likelihood (more information in the manual). But it may be simpler to just encode this expected range in the prior on sigma, rather than placing a prior on two transformations (log(1/sigma)).

Is there a reason you needed this extra prior? Can you try the model without it?

Thank you so much for your information and help.
I want to fit a spatial log-logistic survival model to the data and wrote the likelihood function in stan as user-defined (time and censor indicator variables in the code are respon[i,4] and respon[i,5], respectively).
sigma0 is a hyperparameter and alpha = 1 / sigma is a shape parameter of the log-logistic model.
Given the large dataset and the complexity of our model, what do you suggest about the priors as well as the code modification?

Thanks for the advice!