too many instance performance degradation

I don't remember the exact message, but I got an error when I had more instances of modelsim running than I have cores on my CPU. Do you have extra modelsim simulation windows hanging around? If so close them. Modelsim uses lots of memory. Check you computers available memory against the system requirements for modelsim. You may need more memory if it's running slow. In any case, if your hard disk light is flashing when modelsim simulations are running, you need more memory.