As you know, a single CPU processes one instruction at a time. Adding a second CPU $($or core, in current terminology$)$ allows the system to process two instructions at a time, simultaneously, effectively doubling the processing power of the system. A third core will offer triple the processing power of a single CPU, and so on$.$ However, studies have shown that, in general, the expected increase in computing power starts to decline when the number of cores grows large, beyond eight or so$.$ Why would you expect this to be the case?