How to run Xilinx ISim simulater faster?
You may find another simulator like ghdl (open source) is faster than ISIM for many designs, even though there is less (no) development money available for optimising its performance.

If you structure your testbenches as independent tests, as favoured by Vunit (a VHDL unit testing framework), you can run multiple instances of ghdl (or commercial simulation tools like Modelsim if you can afford enough licenses) in parallel, by simply setting a command line option, up to the number of processors in your system.

(This assumes you're using vhdl, you didn't mention any HDL in your post. I don't know the state of simulation tools for verilog)