R function or package for carrying out maximum likelihood techniques in random effect models

I am applying optim() function in R to obtain maximum likelihood estimates of the fixed effects and random effects in a model with bivariate random effects. The code works well but it is very slow in executing the line of code involving the optim() function. I would like to know if there is/ are any specialized optimization function(s)/ package(s) in R for maximizing likelihoods in random effect models, instead of the general purpose optimization function optim().