The brain has a fixed amount of energy it can use per second, and it takes a fixed amount of energy to erase one bit of information (which you can think of as performing a computation). This means that there are a limited number of computations that the brain can do in one second.

(I'll add that there's no need to make this argument from thermodynamic principles. Your brain has a finite number of neurons, and each neuron can only fire a few times a second, so that limits the number of computations that your brain can do per second as well. And that limit is a lot tighter than the thermodynamic limit.)