If you increase your average speed by 10 km/h, the approximate time gain per 10 kilometres is:
Despite being both dangerous and prohibited, many drivers exceed speed limits in an attempt to save time. However, the actual time gain per 10 kilometres at a speed increase of 10-20 km/h is actually quite small, and continues to decrease the faster you drive.
On the other hand, increasing your speed by 10-20 km/h greatly extends the stopping distance. In other words, by driving too fast, you take a great risk to save a little bit of time.