how to reduce the DDR2 simulation time in modelsim

It all depends how accuratly you are trying to simulate everything. The closer you want to to be to the actual hardware, the longer simulation times are going to be. But if you want really fast, and dont care about propagation delays and such, it might be good to write your own behavioural model of the memory. How you implement this model depends on the usage. if you're only accessing memory sparsly, it can be great to use pointers and only assign memory as its needed.