Lappies aren't designed to run flat out all the time, especially if they cannot keep themselves cool and Apple kit is no exception. Laptops with high-end processors such as the i7 will get hot in such circumstances and having all cores running will only exacerbate matters. Use a system monitor to track the CPU usage. The more cores that are parked, the cooler your laptop will run.
Sweet Thames, run softly till I end my song,
Sweet Thames, run softly, for I speak not loud or long.