If the fans inside your computer are at their highest speeds, then it means your CPU is fully utilized. This usually has an adverse effect on your systems performance, which can be rather annoying, especially if you’re not doing anything on it.
The processor (Central Processing Unit) is essentially the brain of the computer. Like the human brain, it can become overwhelmed, when inundated with too many tasks, or if a task drags on for an extended period of time. Just as things can become more difficult when the human brain is overwhelmed, the processor can grind to a halt, when asked too much of it.