Hi, are you running full throttle or doing little throttling every now and then?
The ESC is a gate that control current to the motor. It has FET ( Field Effect Transistor ) It controls the current to the motor. Lets get simple.
When you full throttle, you are opening the gate wide, so maximum current pass through. The problem come when you throttle a say 25 to 50 percent only. So in theory, you only open the gate about half. Right? So, what about the other half?
It it wasted as heat. That why the FET heat up. So to play save, try not to half throttle longer than necessary. Anyway, these kit are meant to run fast.