When Bluestacks is installed, a good portion of the RAM is assigned to this application, but only after it's started. The application requires a high amount of RAM in order to run things. Since it's an emulator, RAM and CPU go directly one on one. You will see performance decrease if you have a slow PC, in other cases, it simply runs normally.
To increase performance, add more RAM to PC and replace the CPU with a more powerful one.
