The speed of the Thermomix® mixing knife is always gradually increasing from zero to reach the maximum speed. In that way, the mixing knife is not going to push big chunks of food in the upper part of the mixing bowl but cut them into smaller pieces before increase the speed of the knife.
That intelligent way of regulating the speed of the mixing knife assures lower vibrations and even cut pieces.
Sometimes you may notice that as cooking with the Thermomix® the rotation speed of the mixing knife does not increase as you are turning the dial. That behavior is usually consciously programmed in the Thermomix® TM6.
If the speed of the knife in the turbo stage increases only slowly, it could be because the temperature of the pot and its contents is higher than 60 degrees Celsius.
In this case, please let the food cool down and then slowly move the speed selector to level 10.