It's easy to see there must be something wrong with this since if you get 10% faster 8 times, we can be sure that this doesn't mean you're 114% faster (1.1^8 = 2.14). You can't get more than 100% faster!
When you say something is 10% faster, what you mean is it took 10% less time to finish. So 19% is correct.
100 percent faster means doubling the speed,just like a 100 percent salary increase means doubling the salary, or 100 percent car speed increase means doubling its speed.
Unless one uses a bit more esoteric definition of speed, in which a 50 percent of car speed increase, makes it go from 100 km/h to 200 km/h, such that it arrives in half the time.
21%, not 19%.
It is 1.1 * 1.1 = 1.21
You are right in the opposite direction. If it got 10% slower, then it is 0.9 * 0.9 = 0.81 = 19% slower